Home » Public Forums » GMCnet » [GMCnet] Master Cylinders
| [GMCnet] Master Cylinders [message #138245] |
Wed, 10 August 2011 10:22  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Here's some information I just ran down (again!) this morning that some may
find interesting. Mr.erf may want to plug it into the MC table also.
The cross reference application for the GMCMH master cylinder is 76
Chevrolet C20 pickup with 7.4L engine and JB7 brake option. That's for the
1-1/4" diameter bore. The corresponding Wagner number is F79821. AutoZone
number is MN1730, available new or rebuilt. This should also be the same as
the Raybestos 39075 or ACDelco 18M92.
For a 1.125" diameter bore, the same C20 reference can be used, but for JB6
brake option. The AutoZone number is MN1732. Wagner F101267, Raybestos
39009 or ACDelco 18M66
The best I can determine, this is all correct (mostly derived from
RockAuto), but perhaps others should check, because Mr.erf's Master Cylinder
Information table (partially supplied by me) shows the F79821 and 39075 as
1.125" bore. However, on the main Brakes page, 'way down, the 39075 is
described as 1-1/4" bore.
Finally, I'm about to order two of the 79821's from RockAuto on closeout.
$27.22. I was about to order one when I found the shipping for the $7.47
MC would be $12.22. For two MC's the shipping is $12.28. My son and I both
will have spares now. They'll have 7 left at $7.47 after my order.
Oh yeah, how I got onto this: In 2006 when I bought the X-PB, I needed a MC
NOW, so went to AutoZone in Hendersonville, TN and got a MN1730. It's now
giving Alan trouble, so I called AZ here in Americus with my phone number.
They'll have me a NEW replacement MC tomorrow.
Alan & son mostly installed the driver's side quad bag yesterday afternoon.
They'll finish that and the other side tomorrow.
Ken H.
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Ken Henderson
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Master Cylinders [message #138246 is a reply to message #138245] |
Wed, 10 August 2011 10:30  |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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So which one is the best option for our GMC's that gives the best braking???
Ray
Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
----- Original Message ----
From: Ken Henderson <hend4800@bellsouth.net>
To: gmclist <gmclist@temp.gmcnet.org>
Sent: Wed, August 10, 2011 10:22:56 AM
Subject: [GMCnet] Master Cylinders
Here's some information I just ran down (again!) this morning that some may
find interesting. Mr.erf may want to plug it into the MC table also.
The cross reference application for the GMCMH master cylinder is 76
Chevrolet C20 pickup with 7.4L engine and JB7 brake option. That's for the
1-1/4" diameter bore. The corresponding Wagner number is F79821. AutoZone
number is MN1730, available new or rebuilt. This should also be the same as
the Raybestos 39075 or ACDelco 18M92.
For a 1.125" diameter bore, the same C20 reference can be used, but for JB6
brake option. The AutoZone number is MN1732. Wagner F101267, Raybestos
39009 or ACDelco 18M66
The best I can determine, this is all correct (mostly derived from
RockAuto), but perhaps others should check, because Mr.erf's Master Cylinder
Information table (partially supplied by me) shows the F79821 and 39075 as
1.125" bore. However, on the main Brakes page, 'way down, the 39075 is
described as 1-1/4" bore.
Finally, I'm about to order two of the 79821's from RockAuto on closeout.
$27.22. I was about to order one when I found the shipping for the $7.47
MC would be $12.22. For two MC's the shipping is $12.28. My son and I both
will have spares now. They'll have 7 left at $7.47 after my order.
Oh yeah, how I got onto this: In 2006 when I bought the X-PB, I needed a MC
NOW, so went to AutoZone in Hendersonville, TN and got a MN1730. It's now
giving Alan trouble, so I called AZ here in Americus with my phone number.
They'll have me a NEW replacement MC tomorrow.
Alan & son mostly installed the driver's side quad bag yesterday afternoon.
They'll finish that and the other side tomorrow.
Ken H.
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] Master Cylinders [message #138255 is a reply to message #138245] |
Wed, 10 August 2011 10:56 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Senior Member |
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Ken, how did you find these on close-out, it shows $42.79 when I pull it up.
Ray
Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
----- Original Message ----
From: Ken Henderson <hend4800@bellsouth.net>
To: gmclist <gmclist@temp.gmcnet.org>
Sent: Wed, August 10, 2011 10:22:56 AM
Subject: [GMCnet] Master Cylinders
Here's some information I just ran down (again!) this morning that some may
find interesting. Mr.erf may want to plug it into the MC table also.
The cross reference application for the GMCMH master cylinder is 76
Chevrolet C20 pickup with 7.4L engine and JB7 brake option. That's for the
1-1/4" diameter bore. The corresponding Wagner number is F79821. AutoZone
number is MN1730, available new or rebuilt. This should also be the same as
the Raybestos 39075 or ACDelco 18M92.
For a 1.125" diameter bore, the same C20 reference can be used, but for JB6
brake option. The AutoZone number is MN1732. Wagner F101267, Raybestos
39009 or ACDelco 18M66
The best I can determine, this is all correct (mostly derived from
RockAuto), but perhaps others should check, because Mr.erf's Master Cylinder
Information table (partially supplied by me) shows the F79821 and 39075 as
1.125" bore. However, on the main Brakes page, 'way down, the 39075 is
described as 1-1/4" bore.
Finally, I'm about to order two of the 79821's from RockAuto on closeout.
$27.22. I was about to order one when I found the shipping for the $7.47
MC would be $12.22. For two MC's the shipping is $12.28. My son and I both
will have spares now. They'll have 7 left at $7.47 after my order.
Oh yeah, how I got onto this: In 2006 when I bought the X-PB, I needed a MC
NOW, so went to AutoZone in Hendersonville, TN and got a MN1730. It's now
giving Alan trouble, so I called AZ here in Americus with my phone number.
They'll have me a NEW replacement MC tomorrow.
Alan & son mostly installed the driver's side quad bag yesterday afternoon.
They'll finish that and the other side tomorrow.
Ken H.
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] Master Cylinders [message #138256 is a reply to message #138245] |
Wed, 10 August 2011 11:03 |
 |
Michael Leipold
Messages: 318
Registered: April 2011
Location: Greensboro NC
Karma: 2
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Senior Member |
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I just bought one of those Master Cylinders from Rock Auto, probably should have bought two. From what I have read, the 1.125 Bore will give you a higher pressure, which is what you want. I think at some point, they started recommending the 1.25 bore as a replacement and the 1.125's are getting harder to find. Probably due to supply and demand.
The front rubber hoses and the larger rear brake cylinder are also on closeout from Rock Auto.
I have the master cylinder installed, the one I removed was a 1.25 bore and the new master cylinder is physically bigger, but it still fits. I have not bled the rears yet, as it is still up in the air while I do some other work to the front end.
Search for Parts:
Manufacturer: Raybestos
Part Number: MC39009
1973 GMC 26' Glacier - Unknown Mileage - Has a new switch pitch transmission with Powerdrive 
[Updated on: Wed, 10 August 2011 11:07] Report message to a moderator |
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| Re: [GMCnet] Master Cylinders [message #138257 is a reply to message #138255] |
Wed, 10 August 2011 11:06 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Ray, I am not Ken but I am a frequent RockAuto customer. They track the make
& model of your most recent purchases and when they have specials on those
pieces, their computer kicks out the most recent customers and sends out
internet specials. Clever marketers, those folks are.
Jim Hupy
Salem, OR
78 GMc Royale 403
On Wed, Aug 10, 2011 at 8:56 AM, Ray Erspamer <78gmc-royale@att.net> wrote:
> Ken, how did you find these on close-out, it shows $42.79 when I pull it
> up.
>
> Ray
>
>
> Ray & Lisa Erspamer
> 78 Royale "Great Lakes Eagle"
> Center Kitchen TZE368V101144
> Wauwatosa, Wisconsin 53226
> Email: 78GMC-Royale@att.net
> 414-745-3188
> Web Site: http://ray-lisa.page.tl/
>
>
>
> ----- Original Message ----
> From: Ken Henderson <hend4800@bellsouth.net>
> To: gmclist <gmclist@temp.gmcnet.org>
> Sent: Wed, August 10, 2011 10:22:56 AM
> Subject: [GMCnet] Master Cylinders
>
> Here's some information I just ran down (again!) this morning that some
> may
> find interesting. Mr.erf may want to plug it into the MC table also.
>
> The cross reference application for the GMCMH master cylinder is 76
> Chevrolet C20 pickup with 7.4L engine and JB7 brake option. That's for the
> 1-1/4" diameter bore. The corresponding Wagner number is F79821. AutoZone
> number is MN1730, available new or rebuilt. This should also be the same
> as
> the Raybestos 39075 or ACDelco 18M92.
>
> For a 1.125" diameter bore, the same C20 reference can be used, but for JB6
> brake option. The AutoZone number is MN1732. Wagner F101267, Raybestos
> 39009 or ACDelco 18M66
>
> The best I can determine, this is all correct (mostly derived from
> RockAuto), but perhaps others should check, because Mr.erf's Master
> Cylinder
> Information table (partially supplied by me) shows the F79821 and 39075 as
> 1.125" bore. However, on the main Brakes page, 'way down, the 39075 is
> described as 1-1/4" bore.
>
> Finally, I'm about to order two of the 79821's from RockAuto on closeout.
> $27.22. I was about to order one when I found the shipping for the $7.47
> MC would be $12.22. For two MC's the shipping is $12.28. My son and I
> both
> will have spares now. They'll have 7 left at $7.47 after my order.
>
> Oh yeah, how I got onto this: In 2006 when I bought the X-PB, I needed a
> MC
> NOW, so went to AutoZone in Hendersonville, TN and got a MN1730. It's now
> giving Alan trouble, so I called AZ here in Americus with my phone number.
> They'll have me a NEW replacement MC tomorrow.
>
> Alan & son mostly installed the driver's side quad bag yesterday afternoon.
> They'll finish that and the other side tomorrow.
>
> Ken H.
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
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| Re: [GMCnet] Master Cylinders [message #138259 is a reply to message #138246] |
Wed, 10 August 2011 11:10 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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That's one of those "can't be answered" questions. The Master Cylinder
information at gmcmotorhomeinfo.com gives about as much information as
anyone could need to decide, but can't answer the question.
Here are just a few of the points to consider
Small MC bore gives:
Softer pedal (more pedal travel per unit foot pressure)
More maximum braking (more force on the friction surfaces)
Greater chance of running out of pedal (especially with larger
calipers and/or wheel cylinders)
Large MC bore yields just the opposite effect on each of the above.
That last item is the real biggie -- you don't ever want to give out of
pedal! There are a lot of factors affecting that beyond just the sizes of
the calipers and wheel cylinders. Maladjusted drum brakes are the biggest
culprit with that type brakes installed. With disc brakes, other factors
come into play: The characteristics of the calipers themselves (how much
hysteresis there is in their seals, causing retraction of the pistons when
pressure is released); the stability of the caliper mounts (do they allow
the calipers to move other than toward and away from the rotors); the
run-out (warpage) of the rotors, causing the pistons to be pushed back into
the calipers; the looseness of the wheel bearings, allowing the hubs &
rotors to move laterally, pushing the pistons back. And probably others I'm
overlooking.
There are those with 80 mm front calipers and 74 mm center and rear calipers
who can use 1-1/8" MC's. But they must be VERY careful about all of those
tolerances and keep a careful watch on the MC fluid level. They can have
great brakes -- but have little margin for deterioration.
Others with the same brakes except using the big 34 mm P-30 MC,have somewhat
less effective braking, but don't have to worry much about that situation
deteriorating.
Your choice.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
On Wed, Aug 10, 2011 at 11:30 AM, Ray Erspamer <78gmc-royale@att.net> wrote:
> So which one is the best option for our GMC's that gives the best
> braking???
>
>
>
> Ray
>
>
> Ray & Lisa Erspamer
> 78 Royale "Great Lakes Eagle"
> Center Kitchen TZE368V101144
> Wauwatosa, Wisconsin 53226
> Email: 78GMC-Royale@att.net
> 414-745-3188
> Web Site: http://ray-lisa.page.tl/
>
>
>
> ----- Original Message ----
> From: Ken Henderson <hend4800@bellsouth.net>
> To: gmclist <gmclist@temp.gmcnet.org>
> Sent: Wed, August 10, 2011 10:22:56 AM
> Subject: [GMCnet] Master Cylinders
>
> Here's some information I just ran down (again!) this morning that some may
> find interesting. Mr.erf may want to plug it into the MC table also.
>
> The cross reference application for the GMCMH master cylinder is 76
> Chevrolet C20 pickup with 7.4L engine and JB7 brake option. That's for the
> 1-1/4" diameter bore. The corresponding Wagner number is F79821. AutoZone
> number is MN1730, available new or rebuilt. This should also be the same
> as
> the Raybestos 39075 or ACDelco 18M92.
>
> For a 1.125" diameter bore, the same C20 reference can be used, but for JB6
> brake option. The AutoZone number is MN1732. Wagner F101267, Raybestos
> 39009 or ACDelco 18M66
>
> The best I can determine, this is all correct (mostly derived from
> RockAuto), but perhaps others should check, because Mr.erf's Master
> Cylinder
> Information table (partially supplied by me) shows the F79821 and 39075 as
> 1.125" bore. However, on the main Brakes page, 'way down, the 39075 is
> described as 1-1/4" bore.
>
> Finally, I'm about to order two of the 79821's from RockAuto on closeout.
> $27.22. I was about to order one when I found the shipping for the $7.47
> MC would be $12.22. For two MC's the shipping is $12.28. My son and I
> both
> will have spares now. They'll have 7 left at $7.47 after my order.
>
> Oh yeah, how I got onto this: In 2006 when I bought the X-PB, I needed a
> MC
> NOW, so went to AutoZone in Hendersonville, TN and got a MN1730. It's now
> giving Alan trouble, so I called AZ here in Americus with my phone number.
> They'll have me a NEW replacement MC tomorrow.
>
> Alan & son mostly installed the driver's side quad bag yesterday afternoon.
> They'll finish that and the other side tomorrow.
>
> Ken H.
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
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> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Master Cylinders [message #138261 is a reply to message #138255] |
Wed, 10 August 2011 11:19 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Go to 76 Chevy C-20 PU 7.4L/Brake-Hub/MC and it should give a whole bunch of
MC's; the first one should be their P/N
R79821<http://wiki.rockauto.com/1976%20CHEVROLET%20C20%20PICKUP%207.4L%20454cid%20V8%204BBL%20(S)%20OHV_Master%20Cylinder_1032039_1836>,
a manufacturer's closeout with 30 day warranty for $7.47.
You might search by that P/N first.
I may not get the same offers you do since I'm signed up for special offers,
etc. For example, until 7 August, I had a standing 5% discount available
even on those closeout prices.
Ken H.
On Wed, Aug 10, 2011 at 11:56 AM, Ray Erspamer <78gmc-royale@att.net> wrote:
> Ken, how did you find these on close-out, it shows $42.79 when I pull it
> up.
>
> Ray
>
>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Master Cylinders [message #138272 is a reply to message #138268] |
Wed, 10 August 2011 13:14 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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The OEM GMC MC had a 1.125" bore. It can be replaced with either the 1.125"
bore, or the 1.250" bore although the 1.250" bore will have less total
pressure. Approx. 700 psi vs 1000 psi for the 1.125" bore. When you convert
the rear brakes to all discs, the 1.125" bore no longer has the volume of
fluid necessary to operate an all disc system. It might when the rotors and
pads are all in new condition, but as the linings and rotors wear, it
requires more volume than the proper safety margin of reserve fluid in the
resevoirs allows for. Some have used the smaller bore, and by always making
sure that the resevoirs are full, they have been able to manage. The problem
arises with the volume of the cylinder, not the resevoirs. With the all disc
system, you need the larger volume from the larger cylinder bore. As the
discs are much more effective and fade resistant, the lower total system
pressure is not a significant issue. The final answer, if there is such a
thing, is to install the reaction braking system with all discs. Your coach
will stop like a new modern vehicle and will not skid the rear tires. If
there are any doubters out there, just take a ride in Rick Denny's or Dan
Gregg's coaches and have them test the brakes for you. Make sure you are
hanging on to something when they do.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Wed, Aug 10, 2011 at 10:51 AM, bill schurman <lizzie1243@hotmail.com>wrote:
>
> Got mine from Jim B. hope it fits (not worried)
>
>
> William S. Schurman
> P.O. 773325
> Steamboat Springs, CO (Ski Town USA)
> at the foot of Rabbit Ears Pass
> 970-846-4212
> '78 Palm Beach TZE 168V100258
> '90 SHO
> '66 912
>
>
>
>
>
>
>
> > To: gmclist@temp.gmcnet.org
> > From: lilmyk@gmail.com
> > Date: Wed, 10 Aug 2011 12:40:50 -0500
> > Subject: Re: [GMCnet] Master Cylinders
> >
> >
> >
> > From most of what I have found, the Wagner R79821 is actually 1 1/4 bore.
> >
> > I have not physically measured one, but from every other site, that is
> what they are stating.
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
> _______________________________________________
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| Re: [GMCnet] Master Cylinders [message #138283 is a reply to message #138245] |
Wed, 10 August 2011 15:14 |
KB
Messages: 1262
Registered: September 2009
Karma: 0
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Senior Member |
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Thanks Ken for sorting this out. I've been scratching my head over it for a while.
Interestingly, the RockAuto closeout MC only shows up for the '76 truck, not the '75 or '77.
I had planned to go OEM 1.125 for max pressure, but with the new bigger calipers in front on
the 1-ton kit, I'm thinking the closeout 1.25". Don't know if it's necessary,
but at least it's cheap.
Thanks,
Karen
1973 23'
1975 26'
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Karen
1975 26'
San Jose, CA
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| Re: [GMCnet] Master Cylinders [message #138288 is a reply to message #138283] |
Wed, 10 August 2011 15:54 |
Ray Erspamer
Messages: 1707
Registered: May 2007
Location: Milwaukee Wisconsin
Karma: -3
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Senior Member |
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I also grabbed one of these close-out MC's, was a no brainer. Mine has a slow
leak, time to fix it.
Thanks for alerting me of this deal !!
Ray
Ray & Lisa Erspamer
78 Royale "Great Lakes Eagle"
Center Kitchen TZE368V101144
Wauwatosa, Wisconsin 53226
Email: 78GMC-Royale@att.net
414-745-3188
Web Site: http://ray-lisa.page.tl/
----- Original Message ----
From: KB <kab7@sonic.net>
To: gmclist@temp.gmcnet.org
Sent: Wed, August 10, 2011 3:14:12 PM
Subject: Re: [GMCnet] Master Cylinders
Thanks Ken for sorting this out. I've been scratching my head over it for a
while.
Interestingly, the RockAuto closeout MC only shows up for the '76 truck, not the
'75 or '77.
I had planned to go OEM 1.125 for max pressure, but with the new bigger calipers
in front on
the 1-ton kit, I'm thinking the closeout 1.25". Don't know if it's necessary,
but at least it's cheap.
Thanks,
Karen
1973 23'
1975 26'
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Ray Erspamer
78 GMC Royale Center Kitchen
403, 3.70 Final Drive
Holley Sniper Quadrajet EFI System,
Holley Hyperspark Ignition System
414-484-9431
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| Re: [GMCnet] Master Cylinders [message #138301 is a reply to message #138259] |
Wed, 10 August 2011 17:21 |
Mr ERFisher
Messages: 7117
Registered: August 2005
Karma: 2
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Senior Member |
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Look at the pressure calcs and clamping force by Dave m. And the answers become more clear.
Gene
FREE WIFI @ Mickey D
On Aug 10, 2011, at 9:10 AM, Ken Henderson <hend4800@bellsouth.net> wrote:
> That's one of those "can't be answered" questions. The Master Cylinder
> information at gmcmotorhomeinfo.com gives about as much information as
> anyone could need to decide, but can't answer the question.
>
> Here are just a few of the points to consider
>
> Small MC bore gives:
> Softer pedal (more pedal travel per unit foot pressure)
> More maximum braking (more force on the friction surfaces)
> Greater chance of running out of pedal (especially with larger
> calipers and/or wheel cylinders)
>
> Large MC bore yields just the opposite effect on each of the above.
>
> That last item is the real biggie -- you don't ever want to give out of
> pedal! There are a lot of factors affecting that beyond just the sizes of
> the calipers and wheel cylinders. Maladjusted drum brakes are the biggest
> culprit with that type brakes installed. With disc brakes, other factors
> come into play: The characteristics of the calipers themselves (how much
> hysteresis there is in their seals, causing retraction of the pistons when
> pressure is released); the stability of the caliper mounts (do they allow
> the calipers to move other than toward and away from the rotors); the
> run-out (warpage) of the rotors, causing the pistons to be pushed back into
> the calipers; the looseness of the wheel bearings, allowing the hubs &
> rotors to move laterally, pushing the pistons back. And probably others I'm
> overlooking.
>
> There are those with 80 mm front calipers and 74 mm center and rear calipers
> who can use 1-1/8" MC's. But they must be VERY careful about all of those
> tolerances and keep a careful watch on the MC fluid level. They can have
> great brakes -- but have little margin for deterioration.
>
> Others with the same brakes except using the big 34 mm P-30 MC,have somewhat
> less effective braking, but don't have to worry much about that situation
> deteriorating.
>
> Your choice.
>
> Ken H.
> Americus, GA
> '76 X-Birchaven w/Cad500/Howell EFI+ & EBL
> www.gmcwipersetc.com
>
>
>
> On Wed, Aug 10, 2011 at 11:30 AM, Ray Erspamer <78gmc-royale@att.net> wrote:
>
>> So which one is the best option for our GMC's that gives the best
>> braking???
>>
>>
>>
>> Ray
>>
>>
>> Ray & Lisa Erspamer
>> 78 Royale "Great Lakes Eagle"
>> Center Kitchen TZE368V101144
>> Wauwatosa, Wisconsin 53226
>> Email: 78GMC-Royale@att.net
>> 414-745-3188
>> Web Site: http://ray-lisa.page.tl/
>>
>>
>>
>> ----- Original Message ----
>> From: Ken Henderson <hend4800@bellsouth.net>
>> To: gmclist <gmclist@temp.gmcnet.org>
>> Sent: Wed, August 10, 2011 10:22:56 AM
>> Subject: [GMCnet] Master Cylinders
>>
>> Here's some information I just ran down (again!) this morning that some may
>> find interesting. Mr.erf may want to plug it into the MC table also.
>>
>> The cross reference application for the GMCMH master cylinder is 76
>> Chevrolet C20 pickup with 7.4L engine and JB7 brake option. That's for the
>> 1-1/4" diameter bore. The corresponding Wagner number is F79821. AutoZone
>> number is MN1730, available new or rebuilt. This should also be the same
>> as
>> the Raybestos 39075 or ACDelco 18M92.
>>
>> For a 1.125" diameter bore, the same C20 reference can be used, but for JB6
>> brake option. The AutoZone number is MN1732. Wagner F101267, Raybestos
>> 39009 or ACDelco 18M66
>>
>> The best I can determine, this is all correct (mostly derived from
>> RockAuto), but perhaps others should check, because Mr.erf's Master
>> Cylinder
>> Information table (partially supplied by me) shows the F79821 and 39075 as
>> 1.125" bore. However, on the main Brakes page, 'way down, the 39075 is
>> described as 1-1/4" bore.
>>
>> Finally, I'm about to order two of the 79821's from RockAuto on closeout.
>> $27.22. I was about to order one when I found the shipping for the $7.47
>> MC would be $12.22. For two MC's the shipping is $12.28. My son and I
>> both
>> will have spares now. They'll have 7 left at $7.47 after my order.
>>
>> Oh yeah, how I got onto this: In 2006 when I bought the X-PB, I needed a
>> MC
>> NOW, so went to AutoZone in Hendersonville, TN and got a MN1730. It's now
>> giving Alan trouble, so I called AZ here in Americus with my phone number.
>> They'll have me a NEW replacement MC tomorrow.
>>
>> Alan & son mostly installed the driver's side quad bag yesterday afternoon.
>> They'll finish that and the other side tomorrow.
>>
>> Ken H.
>> _______________________________________________
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>>
>> _______________________________________________
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>>
> _______________________________________________
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| Re: [GMCnet] Master Cylinders [message #138341 is a reply to message #138329] |
Wed, 10 August 2011 21:24 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
|
Senior Member |
|
|
Dolph,
That's a vacuum boost pump installed by a PO to power the brake booster in
case of engine failure. It's probably plumbed in series with the vacuum
line from the engine to the booster.
Go here to learn more about them:
http://gmcmotorhomeinfo.com/addens.html#pump
What you have is the older version, now becoming rare and failure prone. It
is NOT the JC4, which is entirely different in design and in current
production.
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
On Wed, Aug 10, 2011 at 9:42 PM, Dolph Santorine
<dolph@dolphsantorine.com>wrote:
> Serendipity is a great thing.... I was looking at this gizmo on the vacuum
> line to the master cylinder:
>
>
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=40112&title=i-don-27t-know-what-this-is&cat=5845
>
> I'm assuming that's an upgraded master cylinder as well. (Next Photo)
>
> Anyone know what it is?
>
>
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Unsubscribe or Change List Options:
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
|
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| Re: [GMCnet] Master Cylinders [message #138343 is a reply to message #138341] |
Wed, 10 August 2011 21:31 |
Dolph Santorine
Messages: 1236
Registered: April 2011
Location: Wheeling, WV
Karma: -41
|
Senior Member |
|
|
Ken:
What's the failure mode? Is there any reason not to use this pump until failure?
Dolph
On Aug 10, 2011, at 10:24 PM, Ken Henderson wrote:
> Dolph,
>
> That's a vacuum boost pump installed by a PO to power the brake booster in
> case of engine failure. It's probably plumbed in series with the vacuum
> line from the engine to the booster.
>
> Go here to learn more about them:
> http://gmcmotorhomeinfo.com/addens.html#pump
>
> What you have is the older version, now becoming rare and failure prone. It
> is NOT the JC4, which is entirely different in design and in current
> production.
>
> Ken H.
> Americus, GA
> '76 X-Birchaven w/Cad500/Howell EFI+ & EBL
> www.gmcwipersetc.com
>
>
>
> On Wed, Aug 10, 2011 at 9:42 PM, Dolph Santorine
> <dolph@dolphsantorine.com>wrote:
>
>> Serendipity is a great thing.... I was looking at this gizmo on the vacuum
>> line to the master cylinder:
>>
>>
>> http://www.gmcmhphotos.com/photos/showphoto.php?photo=40112&title=i-don-27t-know-what-this-is&cat=5845
>>
>> I'm assuming that's an upgraded master cylinder as well. (Next Photo)
>>
>> Anyone know what it is?
>>
>>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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| Re: [GMCnet] Master Cylinders [message #138360 is a reply to message #138343] |
Wed, 10 August 2011 22:11 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
|
Senior Member |
|
|
Dolph,
The original application of most of the pumps had them mounted beneath the
battery box exposed to a lot of road grime. The motor bearings and brushes
were therefore very prone to failure. If it runs quietly, it should be OK.
Probably almost as common was rupture of the diaphragm which moves the air
-- a good backfire could do that immediately. You can easily test that by
depleting the vacuum in the booster, powering up the pump without the engine
running, and see if you get vacuum back.
The original, and probably current, plumbing was in series with the vacuum
from the engine and the pump ran only when the vacuum dropped below some
preset level (10 in Hg ??) and stopped when the vacuum reached 18 in Hg,
IIRC. The switch for that was pretty reliable in my experience, but is
another failure mode.
If it works, keep it.
JWID,
Ken H.
On Wed, Aug 10, 2011 at 10:31 PM, Dolph Santorine
<dolph@dolphsantorine.com>wrote:
> Ken:
>
> What's the failure mode? Is there any reason not to use this pump until
> failure?
>
>
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Unsubscribe or Change List Options:
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
|
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| Re: [GMCnet] Master Cylinders [message #138362 is a reply to message #138360] |
Wed, 10 August 2011 22:14 |
Dolph Santorine
Messages: 1236
Registered: April 2011
Location: Wheeling, WV
Karma: -41
|
Senior Member |
|
|
Thanks, Ken.
Having a second vacuum source calms my inner pilot.
On Aug 10, 2011, at 11:11 PM, Ken Henderson wrote:
> Dolph,
>
> The original application of most of the pumps had them mounted beneath the
> battery box exposed to a lot of road grime. The motor bearings and brushes
> were therefore very prone to failure. If it runs quietly, it should be OK.
>
>
> Probably almost as common was rupture of the diaphragm which moves the air
> -- a good backfire could do that immediately. You can easily test that by
> depleting the vacuum in the booster, powering up the pump without the engine
> running, and see if you get vacuum back.
>
> The original, and probably current, plumbing was in series with the vacuum
> from the engine and the pump ran only when the vacuum dropped below some
> preset level (10 in Hg ??) and stopped when the vacuum reached 18 in Hg,
> IIRC. The switch for that was pretty reliable in my experience, but is
> another failure mode.
>
> If it works, keep it.
>
> JWID,
>
> Ken H.
>
> On Wed, Aug 10, 2011 at 10:31 PM, Dolph Santorine
> <dolph@dolphsantorine.com>wrote:
>
>> Ken:
>>
>> What's the failure mode? Is there any reason not to use this pump until
>> failure?
>>
>>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
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