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| [GMCnet] Brake Fluid Pressures [message #146590] |
Sat, 15 October 2011 09:21  |
Richard Smith
Messages: 14
Registered: February 2004
Karma: 0
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Junior Member |
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GMC'ers
I have always had weak brakes, have made many of the recommended
changes with only little improvements.
Measured the Brake fluid pressures and found:
Front drivers: 800 psi
Rear drivers: 750 psi
The Master Cylinder is a Raybestos MC39075 and the Combo valve is new.
These pressures seem low to me, especially the front.
Is there anyone out there with info on what these pressures should be
and/or what is causing the low pressures??
TIA
Richard 76 Glenbrook
at the Castle in Colorado
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| Re: [GMCnet] Brake Fluid Pressures [message #146611 is a reply to message #146590] |
Sat, 15 October 2011 13:04  |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Richard,
I hate to be the bearer of bad news, but that's probably normal.
Take a look at this spreadsheet by Dave Mumert.
http://www.gmcmotorhome.info/GMCBrakeCalcs.pdf
It covers most of the common MC/Caliper combinations, and shows their
effects. Of particular interest for your immediate question is near the
bottom, titled "Master Cylinder Pressure Available". Notice that the
stipulated pressure on the MC rod is 1000#. I don't remember off-hand the
mechanical advantage of the brake pedal arms, but it's somewhere near 6:1,
so to achieve that 1000 lbf, you'd have to press the brake pedal with
1000/6=167 lbf -- a HARD push even for my 230# self. Your 800 psi says that
with your 1.25" MC, you were pushing on the pedal with (800/1006)*167=133
lbf, still a pretty hard push.
If you prefer empirical data, here's some I gathered back in '03. It's
generally in line with your numbers and the calculations:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=4196&cat=3412
Hydroboost, PowerMaster, or other options are needed to reach the 1000++ psi
we really need -- no known vacuum booster can do it due to the space
limitation of 11" diaphragms.
JWITIK,
Ken H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI+ & EBL
www.gmcwipersetc.com
On Sat, Oct 15, 2011 at 10:21 AM, Richard Smith <richard.smith8@gmail.com>wrote:
> GMC'ers
>
> I have always had weak brakes, have made many of the recommended
> changes with only little improvements.
>
> Measured the Brake fluid pressures and found:
> Front drivers: 800 psi
> Rear drivers: 750 psi
>
> The Master Cylinder is a Raybestos MC39075 and the Combo valve is new.
>
> These pressures seem low to me, especially the front.
>
> Is there anyone out there with info on what these pressures should be
> and/or what is causing the low pressures??
>
> TIA
>
> Richard 76 Glenbrook
> at the Castle in Colorado
>
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www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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| Re: [GMCnet] Brake Fluid Pressures [message #146619 is a reply to message #146611] |
Sat, 15 October 2011 15:43 |
Russell Mehlenbacher
Messages: 128
Registered: June 2010
Karma: 1
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On Sat, Oct 15, 2011 at 2:04 PM, Ken Henderson <hend4800@bellsouth.net>wrote:
> <http://www.gmcmhphotos.com/photos/showphoto.php?photo=4196&cat=3412>
>
> Hydroboost, PowerMaster, or other options are needed to reach the 1000++
> psi
> we really need -- no known vacuum booster can do it due to the space
> limitation of 11" diaphragms.
>
> It was raining this morning, so instead of raking leaves, I was daydreaming
about the 6 wheel disc dilemma. When I brought home the '76 (PO - Mark
Wall), I was not initially impressed with the brakes. They are better when
warmed up a bit, as advertised.
My '74 (8000 miles on "new" parts) stops better under normal use. I have too
many loose objects in either of them to do any panic stops.
I found this: http://www.gorillaperformance.com/
A Hot Rod site, selling dual master cylinders with a mechanical, adjustable
balance bar between them.
"The purpose of this Dual Master Cylinder setup is to get rid of the booster
altogether...",
because the engines they like to use don't make lots of reliable vacuum.
I haven't looked very far into this, I did go deal with the falling leaf
population, but I'm wondering if this has been on the table before.
And I wouldn't need to buy a vacuum pump!
Maybe someone has done this with one of their other toys.
Thoughts, experience, and opinions welcome...
Back to the lawn...
Russ
--
Russ & Linda Mehlenbacher
Rochester Hills, MI
'74 X-Sequoia
'76 Eleganza
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'76 Eleganza
'74 Sequoia
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| Re: [GMCnet] Brake Fluid Pressures [message #146622 is a reply to message #146619] |
Sat, 15 October 2011 18:26 |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma: 9
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Senior Member |
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Russ,
Congratulations on your purchase of Mark's coach. If I'd been in the market
for a 26', you'd probably have missed out.
Don't waste time or money on the "dual MC" idea. The simple physical fact
is that the hydraulic pressure is directly proportional to the force on the
piston(s) and inversely proportional to the effective area of the piston.
In other words, for a given size of slave cylinders, a large MC diameter
will give less pressure for a given force than will a small one.
So why not use a tiny MC diameter to get LOTS of pressure? Because the MC
must supply enough brake fluid volume to move the slave cylinders the
required distance. For a small MC to move enough fluid it must move a LONG
way. The physical constraints of acceptable pedal height, room for the
pedal linkage to move behind the dash, and availability of a small diameter
MC with sufficient stroke all conspire against us.
I wish you luck finding a way to easily increase brake line pressure -- I'll
copy you in a flash. But the only ones I know who've succeeded use
Hydroboost, PowerMaster, or other active (as opposed to vacuum) boost. The
simple physics involved just demand it.
Oh yeah, the added vacuum pump is for redundancy -- NOT to improve maximum
braking.
Ken H.
On Sat, Oct 15, 2011 at 4:43 PM, Russell Mehlenbacher <russmehl@gmail.com>wrote:
> On Sat, Oct 15, 2011 at 2:04 PM, Ken Henderson <hend4800@bellsouth.net
> >wrote:
>
> > <http://www.gmcmhphotos.com/photos/showphoto.php?photo=4196&cat=3412>
> >
> > Hydroboost, PowerMaster, or other options are needed to reach the 1000++
> > psi
> > we really need -- no known vacuum booster can do it due to the space
> > limitation of 11" diaphragms.
> >
> > It was raining this morning, so instead of raking leaves, I was
> daydreaming
> about the 6 wheel disc dilemma. When I brought home the '76 (PO - Mark
> Wall), I was not initially impressed with the brakes. They are better when
> warmed up a bit, as advertised.
> My '74 (8000 miles on "new" parts) stops better under normal use. I have
> too
> many loose objects in either of them to do any panic stops.
> I found this: http://www.gorillaperformance.com/
> A Hot Rod site, selling dual master cylinders with a mechanical, adjustable
> balance bar between them.
> "The purpose of this Dual Master Cylinder setup is to get rid of the
> booster
> altogether...",
> because the engines they like to use don't make lots of reliable vacuum.
> I haven't looked very far into this, I did go deal with the falling leaf
> population, but I'm wondering if this has been on the table before.
> And I wouldn't need to buy a vacuum pump!
> Maybe someone has done this with one of their other toys.
> Thoughts, experience, and opinions welcome...
> Back to the lawn...
>
>
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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] Brake Fluid Pressures [message #146628 is a reply to message #146622] |
Sat, 15 October 2011 19:03 |
 |
USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Russ,
Double Trouble came with six wheel disk brakes, 80 mm calipers on the front and 80 mm Caddy calipers and disks on the rear but no
parking brake which I needed to pass NJ inspection. I had Ken Frey go through the brakes check everything and replace the rear disks
with drums and hook up a parking brake. By adjusting the brake shoes REALLY close to the drums, tightening the cables to within and
inch of their lives, and screwing the knob on the parking brake lever all the way out it passed. I watched the guy in the drivers
seat strain to pull it on just before he tested it!
The P-30 master cylinder failed (fluid running down the front of the booster) and I replaced it. The brakes were OK but nothing to
write home about.
During my first visit to the COOP Resort we:
1) replaced the lines with stainless steel lines
2) replaced all the rubber hoses with braided stainless steel
3) replaced the calipers
4) replaced the front disks
5) installed carbon metallic pads
6) replaced the ancillary vacuum pump
The brakes improved but still noting to write home about.
During my second visit to the COOP Resort we replaced the brake booster with a sensitized one with dual check valves. One hose went
to the intake manifold the second to the vacuum pump. The braking got dramatically better. In fact when Dan Gregg drove Double
Trouble he couldn't believe how well it stopped compared to his pre reaction arm system.
The sensitized booster does not increase the brake line fluid pressure but it sure does reduce the amount of foot pressure required.
I barely have to touch the brakes now to stop.
During our USA tour in 2010 the pins and springs that retained the driver side drum failed and it wiped out the drum which Bob Price
noticed when I pulled into my spot at DuQuoin. Bob Stone gave me used parts to fix it. Being the perfectionist that I am I bought
two new drums, carbon metallic brake shoes and two spring kits all of which will be installed on my next visit to the USA.
Regards,
Rob M.
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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] Brake Fluid Pressures [message #146633 is a reply to message #146622] |
Sat, 15 October 2011 19:46 |
Russell Mehlenbacher
Messages: 128
Registered: June 2010
Karma: 1
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Senior Member |
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Ken,
I probably would have ignored Mark's coach if you hadn't flagged it as a
deal!
Mark's comment that he got better braking if he pulled the parking brake
first got me thinking that there wasn't enough volume reaching the rear
cylinders. How to do it?
I didn't get too involved in the dual MC, other than thinking it was a novel
solution to the lack of vacuum. I suppose if there was a MC for each axle...
what fun to adjust!
I'd started by wondering if some sort of multiplier for the rear axles could
be found and got sidetracked by Hot Rod parts.
Rob,
My new coach has a sensitized booster, TSM rear brakes and 80's on the
front. It does stop well... but I can feel the lack of rear braking.
Having driven a '76 SOB back from Colorado after the rear brakes went to
maybe 5%, (Both feet on the pedal and the end of the parking area coming up
fast.), I want to maximize the effectiveness of what I have.
It may be that after driving it more, things will feel better. I know the
discs still have some polishing to do.
Russ
--
Russ & Linda Mehlenbacher
Rochester Hills, MI
'74 X-Sequoia
'76 Eleganza
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Rochester Hills, Michigan
'76 Eleganza
'74 Sequoia
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| Re: [GMCnet] Brake Fluid Pressures [message #146639 is a reply to message #146633] |
Sat, 15 October 2011 20:18 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Senior Member |
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Russ, pulling the parking brake slightly verifies the fact that the rear
brake shoes need to be adjusted. What I do to adjust them is to raise the
rear wheels all off the ground, (block under the bogies and deflate the air
bags, jack stands before and after the tires before you even think about
crawling under the coach.) and using a brake adjustment tool, tighten the
starwheels until they can not be turned any further. Then use a skinny
conventional screwdriver and reach through the adjuster slot to hold the
self adjuster lever away from the star wheel and back off the star wheel 5
clicks at a time until the tire & wheel can be turned with a minimum amount
of drag, some slight noise is OK. The reason for cranking the starwheels
fully tight is to make sure that the wheel cylinders are fully retracted,
and the other end of the shoes are tight into the adjusters. For the most
part we do not back our GMCs up often enough to keep the self adjusters
doing their job and as the linings wear, the space increases between the
linings and the shoes, and the rear brakes are less effective, and the pedal
travels further down before building pressure in the system. Try adjusting
your brakes and then drive the coach. You will be able to tell the
difference.
Jim Hupy
Salem, OR
78 GMC Roayle 403
On Sat, Oct 15, 2011 at 5:46 PM, Russell Mehlenbacher <russmehl@gmail.com>wrote:
> Ken,
> I probably would have ignored Mark's coach if you hadn't flagged it as a
> deal!
> Mark's comment that he got better braking if he pulled the parking brake
> first got me thinking that there wasn't enough volume reaching the rear
> cylinders. How to do it?
> I didn't get too involved in the dual MC, other than thinking it was a
> novel
> solution to the lack of vacuum. I suppose if there was a MC for each
> axle...
> what fun to adjust!
> I'd started by wondering if some sort of multiplier for the rear axles
> could
> be found and got sidetracked by Hot Rod parts.
>
> Rob,
> My new coach has a sensitized booster, TSM rear brakes and 80's on the
> front. It does stop well... but I can feel the lack of rear braking.
> Having driven a '76 SOB back from Colorado after the rear brakes went to
> maybe 5%, (Both feet on the pedal and the end of the parking area coming up
> fast.), I want to maximize the effectiveness of what I have.
>
> It may be that after driving it more, things will feel better. I know the
> discs still have some polishing to do.
>
> Russ
>
>
>
>
>
>
> --
> Russ & Linda Mehlenbacher
> Rochester Hills, MI
> '74 X-Sequoia
> '76 Eleganza
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
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| Re: [GMCnet] Brake Fluid Pressures [message #146648 is a reply to message #146639] |
Sat, 15 October 2011 21:04 |
Russell Mehlenbacher
Messages: 128
Registered: June 2010
Karma: 1
|
Senior Member |
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Sorry Jim,
6 wheel discs on the new one.. properly adjusted drums on the "old one".
But this has gotten me thinking... I think that the caliper with the parking
brake in the TSM kit is adjustable. Self adjusts when you use the parking
brake. Apply parking brake 30 times.
(Part No. 3110,* *from 79-85 Eldorado, Riviera, and Toronado and 80-85
Seville)
If the discs are far enough out, they can't get to the disc with the
available volume. I can push more fluid somewhere with a second pump of the
pedal.
We shall see.
Russ
On Sat, Oct 15, 2011 at 9:18 PM, James Hupy <jamesh1296@gmail.com> wrote:
> Russ, pulling the parking brake slightly verifies the fact that the rear
> brake shoes need to be adjusted. What I do to adjust them is to raise the
> rear wheels all off the ground, (block under the bogies and deflate the air
> bags, jack stands before and after the tires before you even think about
> crawling under the coach.) and using a brake adjustment tool, tighten the
> starwheels until they can not be turned any further. Then use a skinny
> conventional screwdriver and reach through the adjuster slot to hold the
> self adjuster lever away from the star wheel and back off the star wheel 5
> clicks at a time until the tire & wheel can be turned with a minimum amount
> of drag, some slight noise is OK. The reason for cranking the starwheels
> fully tight is to make sure that the wheel cylinders are fully retracted,
> and the other end of the shoes are tight into the adjusters. For the most
> part we do not back our GMCs up often enough to keep the self adjusters
> doing their job and as the linings wear, the space increases between the
> linings and the shoes, and the rear brakes are less effective, and the
> pedal
> travels further down before building pressure in the system. Try adjusting
> your brakes and then drive the coach. You will be able to tell the
> difference.
> Jim Hupy
> Salem, OR
> 78 GMC Roayle 403
>
>
>
> On Sat, Oct 15, 2011 at 5:46 PM, Russell Mehlenbacher <russmehl@gmail.com
> >wrote:
>
> > Ken,
> > I probably would have ignored Mark's coach if you hadn't flagged it as a
> > deal!
> > Mark's comment that he got better braking if he pulled the parking brake
> > first got me thinking that there wasn't enough volume reaching the rear
> > cylinders. How to do it?
> > I didn't get too involved in the dual MC, other than thinking it was a
> > novel
> > solution to the lack of vacuum. I suppose if there was a MC for each
> > axle...
> > what fun to adjust!
> > I'd started by wondering if some sort of multiplier for the rear axles
> > could
> > be found and got sidetracked by Hot Rod parts.
> >
> > Rob,
> > My new coach has a sensitized booster, TSM rear brakes and 80's on the
> > front. It does stop well... but I can feel the lack of rear braking.
> > Having driven a '76 SOB back from Colorado after the rear brakes went to
> > maybe 5%, (Both feet on the pedal and the end of the parking area coming
> up
> > fast.), I want to maximize the effectiveness of what I have.
> >
> > It may be that after driving it more, things will feel better. I know the
> > discs still have some polishing to do.
> >
> > Russ
> >
> >
> >
> >
> >
> >
> > --
> > Russ & Linda Mehlenbacher
> > Rochester Hills, MI
> > '74 X-Sequoia
> > '76 Eleganza
> > _______________________________________________
> > GMCnet mailing list
> > Unsubscribe or Change List Options:
> > http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
> >
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Russ & Linda Mehlenbacher
Rochester Hills, MI
'74 X-Sequoia
'76 Eleganza
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Rochester Hills, Michigan
'76 Eleganza
'74 Sequoia
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| Re: [GMCnet] Brake Fluid Pressures [message #146653 is a reply to message #146648] |
Sat, 15 October 2011 21:27 |
|
USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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Senior Member |
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Russ,
Some comments:
1) Are the TSM calipers on the rear 80mm? The reason I ask is the PO of The Blue Streak bought a set from them and gave them to me
when I bought it. They are the 60mm calipers which are used on the Camaro if I'm not mistaken.
2) Which calipers are actuated by the hand brake?
3) It has been reported here that the hand brake mechanism can cause the inner pad to stay in contact with the disk and burn up the
caliper.
Regards,
Rob M.
-----Original Message-----
From: Russell Mehlenbacher
Sorry Jim,
6 wheel discs on the new one.. properly adjusted drums on the "old one".
But this has gotten me thinking... I think that the caliper with the parking
brake in the TSM kit is adjustable. Self adjusts when you use the parking
brake. Apply parking brake 30 times.
(Part No. 3110,* *from 79-85 Eldorado, Riviera, and Toronado and 80-85
Seville)
If the discs are far enough out, they can't get to the disc with the
available volume. I can push more fluid somewhere with a second pump of the
pedal.
We shall see.
Russ
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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] Brake Fluid Pressures [message #146654 is a reply to message #146653] |
Sat, 15 October 2011 21:49 |
Russell Mehlenbacher
Messages: 128
Registered: June 2010
Karma: 1
|
Senior Member |
|
|
Rob,
I'm not sure of the rear calipers yet.
Mark has a write up here:
http://gmcmotorhome.info/master%20cyl.html
http://goo.gl/rKAxV
At the bottom of the page.
I haven't made time to stick my head behind the wheels.
Realistically, these should work well, I'll have to keep an eye on the
adjusters!
Russ
On Sat, Oct 15, 2011 at 10:27 PM, Rob Mueller <robmueller@iinet.net.au>wrote:
> Russ,
>
> Some comments:
>
> 1) Are the TSM calipers on the rear 80mm? The reason I ask is the PO of The
> Blue Streak bought a set from them and gave them to me
> when I bought it. They are the 60mm calipers which are used on the Camaro
> if I'm not mistaken.
>
> 2) Which calipers are actuated by the hand brake?
>
> 3) It has been reported here that the hand brake mechanism can cause the
> inner pad to stay in contact with the disk and burn up the
> caliper.
>
> Regards,
> Rob M.
>
>
> -----Original Message-----
> From: Russell Mehlenbacher
>
> Sorry Jim,
> 6 wheel discs on the new one.. properly adjusted drums on the "old one".
> But this has gotten me thinking... I think that the caliper with the
> parking
> brake in the TSM kit is adjustable. Self adjusts when you use the parking
> brake. Apply parking brake 30 times.
> (Part No. 3110,* *from 79-85 Eldorado, Riviera, and Toronado and 80-85
> Seville)
> If the discs are far enough out, they can't get to the disc with the
> available volume. I can push more fluid somewhere with a second pump of
> the
> pedal.
> We shall see.
>
> Russ
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>
--
Russ & Linda Mehlenbacher
Rochester Hills, MI
'74 X-Sequoia
'76 Eleganza
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
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Russ & Linda Mehlenbacher
Rochester Hills, Michigan
'76 Eleganza
'74 Sequoia
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