Home » Public Forums » GMCnet » [GMCnet] Master Cylinder Push Rod
| [GMCnet] Master Cylinder Push Rod [message #77670] |
Mon, 22 March 2010 14:47  |
Gary Bovee
Messages: 177
Registered: August 2008
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Can anyone tell me what is the correct push rod length that presses against
the P30 master cylinder for those of us who have the Cadillac (1976-1978)
rear disc brake conversion kit installed on their coach?
Thanks for any help.
Gary Bovee
Red Bluff, CA
78 Royale
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| Re: [GMCnet] Master Cylinder Push Rod [message #77821 is a reply to message #77713] |
Wed, 24 March 2010 08:25  |
Jim Bounds
Messages: 842
Registered: January 2004
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Something we recently learned from a hard knock trying to get a modified brake system to work right. The length of the small rod coming out of the booster must not contact the piston in the booster. Then you say why is it, aren't we using the right master cylinder. can't answer your question-- I will tell you it just cannot so when you stab the master onto the booster, leave the outlet ports on the master open. If you get a squirt of fluid as the master bottoms out on the booster (no matter what master you use) you need to resize the output shft slightly. In general we find as much as 1/16" makes a huge difference. On coaches with rear calipers, this we have found is a critical issue. This is not SOP when we install any master to any coach. We have found most install need this output shaft needs to be resized.
This information was very expensive to figure out and valuable. You can do with it what you like-- I give it to you for free,
Jim Bounds
----------------------------
----- Original Message ----
From: Ken Henderson <ken0henderson@gmail.com>
To: gmclist@temp.gmcnet.org
Sent: Tue, March 23, 2010 12:08:33 AM
Subject: Re: [GMCnet] Master Cylinder Push Rod
Gary,
I don't recall the exact length I used, but the note near the center of this
spreadsheet may be helpful:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=2684&cat=3272
<http://www.gmcmhphotos.com/photos/showphoto.php?photo=2684&cat=3272>The
best advice, however, is that you should check the clearance of YOUR system
to be sure all variables are taken into account.
By the way, it's only the change of the MC that affects the push rod length
-- the brakes downstream have no affect on that parameter.
Ken H.
On Mon, Mar 22, 2010 at 3:47 PM, Gary Bovee <gcbgold@digitalpath.net> wrote:
> Can anyone tell me what is the correct push rod length that presses against
> the P30 master cylinder for those of us who have the Cadillac (1976-1978)
> rear disc brake conversion kit installed on their coach?
>
>
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| Re: [GMCnet] Master Cylinder Push Rod [message #77822 is a reply to message #77821] |
Wed, 24 March 2010 08:48 |
Carleton Douglas[1]
Messages: 174
Registered: March 2006
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When they get hot it grow and will put the breaks on as you are going
down the road, need space between the push rod and the master
cylinder, about 1/16" at the most. How long use your math as each one
is machined different and from China.
On Wed, Mar 24, 2010 at 6:25 AM, Jim Bounds <gmccoop@yahoo.com> wrote:
> Something we recently learned from a hard knock trying to get a modified brake system to work right. The length of the small rod coming out of the booster must not contact the piston in the booster. Then you say why is it, aren't we using the right master cylinder. can't answer your question-- I will tell you it just cannot so when you stab the master onto the booster, leave the outlet ports on the master open. If you get a squirt of fluid as the master bottoms out on the booster (no matter what master you use) you need to resize the output shft slightly. In general we find as much as 1/16" makes a huge difference. On coaches with rear calipers, this we have found is a critical issue. This is not SOP when we install any master to any coach. We have found most install need this output shaft needs to be resized.
>
> This information was very expensive to figure out and valuable. You can do with it what you like-- I give it to you for free,
>
> Jim Bounds
> ----------------------------
>
>
>
> ----- Original Message ----
> From: Ken Henderson <ken0henderson@gmail.com>
> To: gmclist@temp.gmcnet.org
> Sent: Tue, March 23, 2010 12:08:33 AM
> Subject: Re: [GMCnet] Master Cylinder Push Rod
>
> Gary,
>
> I don't recall the exact length I used, but the note near the center of this
> spreadsheet may be helpful:
>
> http://www.gmcmhphotos.com/photos/showphoto.php?photo=2684&cat=3272
>
> <http://www.gmcmhphotos.com/photos/showphoto.php?photo=2684&cat=3272>The
> best advice, however, is that you should check the clearance of YOUR system
> to be sure all variables are taken into account.
>
> By the way, it's only the change of the MC that affects the push rod length
> -- the brakes downstream have no affect on that parameter.
>
> Ken H.
>
>
> On Mon, Mar 22, 2010 at 3:47 PM, Gary Bovee <gcbgold@digitalpath.net> wrote:
>
>> Can anyone tell me what is the correct push rod length that presses against
>> the P30 master cylinder for those of us who have the Cadillac (1976-1978)
>> rear disc brake conversion kit installed on their coach?
>>
>>
> _______________________________________________
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--
Carleton Douglas
73 custom, by myself
Prescott, AZ
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| Re: [GMCnet] Master Cylinder Push Rod [message #77836 is a reply to message #77821] |
Wed, 24 March 2010 11:31 |
Gary Worobec
Messages: 867
Registered: May 2005
Karma: -1
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Jim, Thank you for that insight. You take the time and money to figure this
stuff out and you really don't have to let everyone know. However, without
guys like you...guys like me would be up the creek without a paddle.
Thanks
Gary and Joanne Worobec
1973 GMC Glacier
Anza, CA
----- Original Message -----
From: "Jim Bounds" <gmccoop@yahoo.com>
To: <gmclist@temp.gmcnet.org>
Sent: Wednesday, March 24, 2010 6:25 AM
Subject: Re: [GMCnet] Master Cylinder Push Rod
Something we recently learned from a hard knock trying to get a modified
brake system to work right. The length of the small rod coming out of the
booster must not contact the piston in the booster. Then you say why is it,
aren't we using the right master cylinder. can't answer your question-- I
will tell you it just cannot so when you stab the master onto the booster,
leave the outlet ports on the master open. If you get a squirt of fluid as
the master bottoms out on the booster (no matter what master you use) you
need to resize the output shft slightly. In general we find as much as 1/16"
makes a huge difference. On coaches with rear calipers, this we have found
is a critical issue. This is not SOP when we install any master to any
coach. We have found most install need this output shaft needs to be
resized.
This information was very expensive to figure out and valuable. You can do
with it what you like-- I give it to you for free,
Jim Bounds
----------------------------
----- Original Message ----
From: Ken Henderson <ken0henderson@gmail.com>
To: gmclist@temp.gmcnet.org
Sent: Tue, March 23, 2010 12:08:33 AM
Subject: Re: [GMCnet] Master Cylinder Push Rod
Gary,
I don't recall the exact length I used, but the note near the center of this
spreadsheet may be helpful:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=2684&cat=3272
<http://www.gmcmhphotos.com/photos/showphoto.php?photo=2684&cat=3272>The
best advice, however, is that you should check the clearance of YOUR system
to be sure all variables are taken into account.
By the way, it's only the change of the MC that affects the push rod length
-- the brakes downstream have no affect on that parameter.
Ken H.
On Mon, Mar 22, 2010 at 3:47 PM, Gary Bovee <gcbgold@digitalpath.net> wrote:
> Can anyone tell me what is the correct push rod length that presses
> against
> the P30 master cylinder for those of us who have the Cadillac (1976-1978)
> rear disc brake conversion kit installed on their coach?
>
>
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| Re: [GMCnet] Master Cylinder Push Rod [message #77909 is a reply to message #77821] |
Wed, 24 March 2010 17:35 |
 |
USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Jim,
This info is going to help us when I bring Double Trouble to your shop in
June to install one of your new sensitized boosters!
When I replaced the leaking P-30 up in Pennsylvania I found that whoever had
installed the Caddy brakes epoxied a small 8/32 nut to the push rod to
extend it!
Thanks,
Rob Mueller
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Jim Bounds
Sent: Thursday, 25 March 2010 12:25 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Master Cylinder Push Rod
Something we recently learned from a hard knock trying to get a modified
brake system to work right. The length of the small rod coming out of the
booster must not contact the piston in the booster. Then you say why is it,
aren't we using the right master cylinder. can't answer your question-- I
will tell you it just cannot so when you stab the master onto the booster,
leave the outlet ports on the master open. If you get a squirt of fluid as
the master bottoms out on the booster (no matter what master you use) you
need to resize the output shft slightly. In general we find as much as
1/16" makes a huge difference. On coaches with rear calipers, this we have
found is a critical issue. This is not SOP when we install any master to
any coach. We have found most install need this output shaft needs to be
resized.
This information was very expensive to figure out and valuable. You can do
with it what you like-- I give it to you for free,
Jim Bounds
----------------------------
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Master Cylinder Push Rod [message #77942 is a reply to message #77909] |
Wed, 24 March 2010 22:36 |
jimk
Messages: 6734
Registered: July 2006
Location: Belmont, CA
Karma: 9
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Not good.
On Wed, Mar 24, 2010 at 3:35 PM, Rob Mueller <robmueller@iinet.net.au> wrote:
> Jim,
>
> This info is going to help us when I bring Double Trouble to your shop in
> June to install one of your new sensitized boosters!
>
> When I replaced the leaking P-30 up in Pennsylvania I found that whoever had
> installed the Caddy brakes epoxied a small 8/32 nut to the push rod to
> extend it!
>
> Thanks,
> Rob Mueller
> Sydney, Australia
> AUS '75 Avion-The Blue Streak TZE365V100428
> USA '75 Avion-Double Trouble TZE365V100426
>
>
> -----Original Message-----
> From: gmclist-bounces@temp.gmcnet.org
> [mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Jim Bounds
> Sent: Thursday, 25 March 2010 12:25 AM
> To: gmclist@temp.gmcnet.org
> Subject: Re: [GMCnet] Master Cylinder Push Rod
>
> Something we recently learned from a hard knock trying to get a modified
> brake system to work right. The length of the small rod coming out of the
> booster must not contact the piston in the booster. Then you say why is it,
> aren't we using the right master cylinder. can't answer your question-- I
> will tell you it just cannot so when you stab the master onto the booster,
> leave the outlet ports on the master open. If you get a squirt of fluid as
> the master bottoms out on the booster (no matter what master you use) you
> need to resize the output shft slightly. In general we find as much as
> 1/16" makes a huge difference. On coaches with rear calipers, this we have
> found is a critical issue. This is not SOP when we install any master to
> any coach. We have found most install need this output shaft needs to be
> resized.
>
> This information was very expensive to figure out and valuable. You can do
> with it what you like-- I give it to you for free,
>
> Jim Bounds
> ----------------------------
>
> _______________________________________________
> GMCnet mailing list
> List Information and Subscription Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Jim Kanomata
Applied/GMC, Fremont,CA
jimk@appliedairfilters.com
http://www.appliedgmc.com
1-800-752-7502
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Jim Kanomata
Applied/GMC
jimk@appliedairfilters.com
www.appliedgmc.com
1-800-752-7502
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| Re: [GMCnet] Master Cylinder Push Rod [message #78016 is a reply to message #77821] |
Thu, 25 March 2010 11:50 |
 |
mike miller
Messages: 3576
Registered: February 2004
Location: Hillsboro, Oregon
Karma: 0
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Senior Member |
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What Jim says (below) got me thinking. (Dangerous, I know)
When I first got my '73, I needed to replace the MC. I figured I was going to put bigger calipers and rear disks on it so I got a LARGER master cylinder. I didn't put any fluid in it while I checked for fit. I measured how far the push rod went into the old MC and compared that measurement to how far the rod went into the new one. From this information I made a new rod from a stainless steel bolt, honed down to size in a drill with sandpaper.
I didn't like how the larger MC fit (the hatch wouldn't close) so I returned it so I never found out how well (or not) my homemade rod worked.
Now here is my idea:
It wouldn't be that hard to make a slightly to-long rod. Using bench bleeding fittings on a new Master cylinder, test fit the MC and rod. Keep cutting (or sanding) off some until the MC doesn't "squirt" as the MC is pressed "home."
It is just a thought... 
| Jim Bounds wrote on Wed, 24 March 2010 06:25 |
Something we recently learned from a hard knock trying to get a modified brake system to work right. The length of the small rod coming out of the booster must not contact the piston in the booster. Then you say why is it, aren't we using the right master cylinder. can't answer your question-- I will tell you it just cannot so when you stab the master onto the booster, leave the outlet ports on the master open. If you get a squirt of fluid as the master bottoms out on the booster (no matter what master you use) you need to resize the output shft slightly. In general we find as much as 1/16" makes a huge difference. On coaches with rear calipers, this we have found is a critical issue. This is not SOP when we install any master to any coach. We have found most install need this output shaft needs to be resized.
This information was very expensive to figure out and valuable. You can do with it what you like-- I give it to you for free,
Jim Bounds
|
Mike Miller -- Hillsboro, OR -- on the Black list
(#2)`78 23' Birchaven Rear Bath -- (#3)`77 23' Birchaven Side Bath
More Sidekicks than GMC's and a late model Malibu called 'Boo'
http://m000035.blogspot.com
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| Re: [GMCnet] Master Cylinder Push Rod [message #78049 is a reply to message #78043] |
Thu, 25 March 2010 20:13 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Ken,
Good point on the radius!
Regards,
Rob Mueller
Sydney, Australia
AUS '75 Avion-The Blue Streak TZE365V100428
USA '75 Avion-Double Trouble TZE365V100426
-----Original Message-----
From: gmclist-bounces@temp.gmcnet.org
[mailto:gmclist-bounces@temp.gmcnet.org] On Behalf Of Ken Henderson
Sent: Friday, 26 March 2010 10:48 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] Master Cylinder Push Rod
I sure agree with you 100%, Jim. There are too many variables to depend on
anyone's dimensions. Even the radius of the socket in the MC and on the end
of the push rod affect the effective length, making any micrometer reading
essentially meaningless when either changes.
Ken H.
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] Master Cylinder Push Rod [message #89325 is a reply to message #77670] |
Sun, 20 June 2010 15:13 |
James Moore
Messages: 71
Registered: January 2004
Karma: 0
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Member |
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Gene,
I am looking at everything I can think of in an effort to get my brakes
back to where they were when I purchased my GMC and before I started
installing all the so called improvements commonly done to GMC brakes.
Gary Bovee sent me this link which came from Emory Stora and I got a lot of
good information out of it. http://www.autoshop101.com/forms/brake05.pdf
However because of the ball end on the push rods the math didn't work
exactly as shown.
Your website shows the length of the OEM pushrod as 2.469 inches. The
length of my OEM push rod is 2.477 inches.
When I filled the back of the primary piston with putty and pushed the
master cylinder against the booster flange, the push rod barely dented the
putty. I removed the putty and pulled the push rod out until it was even
with the booster flange and pushed the master cylinder against the flange.
I then used a straight edge and a dial caliper to measure the distance from
the booster flange to the end to the push rod and then pushed the pushrod
all the way into the booster and measured the distance from the flange to
the end of the rod. The difference in the two readings is the clearance
between the push rod and the master cylinder primary piston. I repeated
the exercise several times with the same results. The clearance between my
OEM pushrod and the piston was 0.143 inches.
The link Emory sent Gary calls for maximum of 0.020 inches of clearance. I
made a temporary push rod out of a 3/8" bolt with a length of 2.612 inches
and installed it. When I put putty in the end of the primary piston and
pushed the master cylinder against the booster flange, the rod made a deep
indention in the putty but didn't break thru. When I repeated the
measurements, I came up with a clearance of 0.008 inches.
I started the engine and put the tranny in drive. When I revved the engine
to 2000 rpm, the GMC did not move. In the past when I did this, the GMC
would creep forward. I backed up to the end of my dirt drive and
accelerated forward and performed a panic stop. All 4 rear wheels locked
and skidded several inches. Couldn't tell about the front wheels.
Repeated the manuver several times with the same results. I have never
skidded my rear wheels before.
Until I do a road test, I can't say the brakes are improved. However the
results so far are encouraging.
Jim Moore
75 PB
Battle Ground, WA
> [Original Message]
> From: Mr.erf ERFisher <mr.erfisher@gmail.com>
> To: <gmclist@temp.gmcnet.org>
> Date: 6/17/2010 2:57:28 AM
> Subject: Re: [GMCnet] Master Cylinder Push Rod
>
> ahhhh yes, a continuing saga
>
> this depends upon the master cyl you are using, and how you mount the
master
> cyl. Here are some
>
> http://gmcmotorhome.info/master%20cyl.html
>
> what was the old one? and what was the master cyl?
>
> gene
>
>
>
> On Thu, Jun 17, 2010 at 12:05 AM, James Moore
<j.moore.jr@mindspring.com>wrote:
>
> > Does anyone know the length of the push rod between the brake booster
and
> > the master cylinder?
> >
> > Thanks
> >
> > Jim Moore
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