Home » Public Forums » GMCnet » Re: [GMCnet] bad master
| Re: [GMCnet] bad master [message #69752] |
Wed, 06 January 2010 00:17  |
James Moore
Messages: 71
Registered: January 2004
Karma: 0
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I have been monitoring this thread to see if it might give me a clue as how
to fix my lousy brakes. It has given me some things to look at.
When I picked up my 75 Palm Beach, it had been sitting for close to 15 yrs.
I purchased it in PA and immediately drove it to east TX and had no problem
with the brakes. In TX, I replaced the wheel cylinders with the larger
middle ones recommended on this site along with replacing the rear brake
shoes and front pads. I had good brakes.
After driving the GMC to WA via middle GA and the FL panhandle, I started
to improve the brakes. I replaced the master cylinder with one for a P30
having a 1 5/16" bore, replaced the combination valve with a brass one,
installed larger front calipers, replaced all the brake lines and hoses,
and installed a backup vacuum pump. When I finished, I had lousy brakes.
The front wheels would not lock up in gravel. I installed a Powermaster
and it was a slight improvement until it failed but still not as good as
the original brakes.
I had the original booster rebuilt and re-installed it. No improvement. I
checked the vacuum to the booster and found the vacuum pump restricted the
vacuum supply from the manifold so I lost boost if I applied the brakes
several times in a short time. I removed the vacuum pump. No improvement
but I have boost all the time the engine is running. I installed the
original front calipers, a master cylinder with a 1 1/4" bore, 15/16" rear
wheel cylinders and improved the brakes slightly but they are still lousy.
While doing this, I have bled several gallons of brake fluid through the
system and installed numerous master cylinders all to no avail. I tested
the hydraulic system using a high pressure gauge and the maximum pressur I
could develop in the front calipers was less than 800 psig. 1200+ psig is
necessary for good brakes. Got the same results with both 1 1/4" and 1
5/16" master cylinders. Nothing I did would increase the pressure. I
think Arch got about the same result and decided to go to the sticky front
pads.
When I had the local GM dealer cross the combination valve part number over
to the current part number in 2007, it was not the same part number as the
one I have installed. Unfortunately, the original combination valve is not
available from GM. I don't know if this is the cause of my lousy brakes or
not.
Looking at the details on the master cylinder installed on Dan's GMC. I see
the front brakes are connected to the primary or rear connection. Looking
on the Rock Auto site, I see some of the master cylinders used on C30's
have the rear brakes connected to the primary connection. My 78 Blazer
has the rear brakes connected to the primary connection. When I cross the
master cylinder from the parts book to the current part numbers, I come up
with the Dorman #101266 listed in Rock's catalog as used on C30's. However
there is no information on the size of the primary and secondary
connections. Does anyone know if the front brakes connect to the primary
or secondary ports on the master cylinder?
Some more information for Ken. Both 1 1/4" and 1 5/16" master cylinders
are used on C30's. There are two different sizes of brake shoes and wheel
cylinders used. 13 X 2.5" and 13 X 3.5" shoes and 1 1/8" and 1 3/16"
cylinders. The 1 1/4" master cylinder is used with the 13 X 2.5" shoes and
1 1/8" wheel cylinders.
Jim Moore
75 PB
Battle Ground, WA
> [Original Message]
> From: Bob de Kruyff <NEXT2POOL@AOL.COM>
> To: <gmclist@temp.gmcnet.org>
> Date: 1/5/2010 6:00:10 PM
> Subject: Re: [GMCnet] bad master
>
>
>
> ""Yah--but the lack of time!! I think Ken H is the perfect brake tester
here!! Did you notice how he dangled the worm out there--he's trapped a few
of us before! ""
>
> Ken--you are very quiet all of a sudden :)
> --
> Bob de Kruyff
> 78 Eleganza
> Chandler, AZ
> _______________________________________________
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| Re: [GMCnet] bad master [message #69767 is a reply to message #69752] |
Wed, 06 January 2010 07:05  |
GMCWiperMan
Messages: 1248
Registered: December 2007
Karma: 1
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Senior Member |
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Jim,
You've certainly "been the route", haven't you? I'm surprised you haven't
replaced the combination valve before now. If it's like the one I
disassembled, there could be enough crud in there to prevent full pressure
from reaching the front brakes. JimK has a suitable combination valve.
A day or two ago Gene posted the link to the MC table on his site which
shows the port arrangement for several different MC's. OEM did have the
front brakes on the rear piston of the MC. The P-30 for sure has the
fittings reversed. I've swapped MC's so many times that I finally put
couplings just below the vibration loops in both lines so I can swap the
positions easily -- it's also a convenient location to replace a coupling
with a tee for a line pressure gauge.
If you figure out how to get that 1200 psi+ that we DO need, without an
exotic booster, PLEASE let me know. During all the testing I've done, I've
never seen over 1100, IIRC.
Thanks for the C-30 brake info; where did you find it? I have not yet
located a good source for that type of information (mostly wait for Dave
Mumert to do the research). Without running the numbers, it looks as if our
guess was correct: the larger bore MC was installed to keep the line about
the same when they installed the larger wheel cylinders.
Ken H.
Americus, GA
'76 X-Birchaven
www.gmcwipersetc.com
On Wed, Jan 6, 2010 at 1:17 AM, James Moore <j.moore.jr@mindspring.com>wrote:
> ...
> Some more information for Ken. Both 1 1/4" and 1 5/16" master cylinders
> are used on C30's. There are two different sizes of brake shoes and wheel
> cylinders used. 13 X 2.5" and 13 X 3.5" shoes and 1 1/8" and 1 3/16"
> cylinders. The 1 1/4" master cylinder is used with the 13 X 2.5" shoes and
> 1 1/8" wheel cylinders.
>
> Jim Moore
> 75 PB
> Battle Ground, WA
>
>
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| Re: [GMCnet] bad master [message #69816 is a reply to message #69767] |
Wed, 06 January 2010 15:29 |
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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,
Double Trouble has a P-30 MS; Caddy calipers on the front and middle axles
and drums on the rear. It also has a vacuum boost pump.
I when I installed the SS brake line kit I screwed the lines into the ports
they fit in on the MS. The PO had removed the proportioning valve and I
installed a new one because the lines in the kit were bent to attach to one
and because I wanted the brake warning light.
IIRC the proportioning valve sends pressure to the rear cylinders first then
to the fronts and since I've reversed them that would reverse the sequence?
I'm confused!
Did I screw up?
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: Thursday, 7 January 2010 12:06 AM
To: gmclist
Subject: Re: [GMCnet] bad master
Jim,
A day or two ago Gene posted the link to the MC table on his site which
shows the port arrangement for several different MC's. OEM did have the
front brakes on the rear piston of the MC. The P-30 for sure has the
fittings reversed. I've swapped MC's so many times that I finally put
couplings just below the vibration loops in both lines so I can swap the
positions easily -- it's also a convenient location to replace a coupling
with a tee for a line pressure gauge.
Ken H.
Americus, GA
'76 X-Birchaven
www.gmcwipersetc.com
_______________________________________________
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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] bad master [message #69821 is a reply to message #69816] |
Wed, 06 January 2010 15:59 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""IIRC the proportioning valve sends pressure to the rear cylinders first then
to the fronts and since I've reversed them that would reverse the sequence?""
Not Ken here, but the sequence will still be the same since the output lines are still the same.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] bad master [message #69830 is a reply to message #69821] |
Wed, 06 January 2010 18:53 |
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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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Bob,
Thanks! I DO NOT DOUBT what you have noted but I'm still confused!
The reason I'm confused is because of the write-up in the Maintenance Manual
X-7525. On page 5-2 paragraph entitled MASTER CYLINDER it says:
DESCRIPTION
The dual master cylinder is designed so that then front and rear brakes have
separate hydraulic systems. The hydraulic pressure developed in both systems
is equal at all times since the front piston is balance between the
hydraulic pressure in each system.
OPERATION
Fluid is directed to the wheel through two hydraulic outlets, one for the
front brakes and one for the rear brakes.
On page 5-3 the Maintenance Manual says:
METERING VALVE FUNCTION
The metering section of the combination valve operates to "hold off"
hydraulic flow (pressure) until about 130 psi has been built up in the
system before applying the front brakes. The pressure then blends to full
line pressure at approx. 400-600 psi. There is no flow restriction when the
brakes are released.
I can't picture how it does this correctly if the inlet lines are reversed
taking into consideration the information on page 5-2 as noted above.
Yeah, I know Jethro Tull got it right "thick as a brick!"
The Maintenance Manual Are there any pictures / schematics of the
combination valve that show how it works?
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 Bob de Kruyff
Sent: Thursday, 7 January 2010 8:59 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] bad master
""IIRC the proportioning valve sends pressure to the rear cylinders first
then
to the fronts and since I've reversed them that would reverse the
sequence?""
Not Ken here, but the sequence will still be the same since the output lines
are still the same.
--
Bob de Kruyff
78 Eleganza
Chandler, AZ
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
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_______________________________________________
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List Information and Subscription Options:
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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] bad master [message #69836 is a reply to message #69830] |
Wed, 06 January 2010 19:30 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""Bob,
Thanks! I DO NOT DOUBT what you have noted but I'm still confused!
The reason I'm confused is because of the write-up in the Maintenance Manual
X-7525. On page 5-2 paragraph entitled MASTER CYLINDER it says:
DESCRIPTION
The dual master cylinder is designed so that then front and rear brakes have
separate hydraulic systems. The hydraulic pressure developed in both systems
is equal at all times since the front piston is balance between the
hydraulic pressure in each system.
OPERATION
Fluid is directed to the wheel through two hydraulic outlets, one for the
front brakes and one for the rear brakes.""
I understand the confusion--but the only thing that is different is that the front and rear brakes are recieving their respective inputs (both at the same pressure) from different reservoirs--the combo or metering valve doesn't know were it's input comes from--only were it goes to.
Let's keep talking.
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] bad master [message #69838 is a reply to message #69830] |
Wed, 06 January 2010 19:31 |
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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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Bob,
Nevermind, I did a search of the photo site and found this:
http://www.gmcmhphotos.com/photos/showphoto.php?photo=5314&cat=3510
Which Ken posted and I now understand how the combination valve works. The
130 psi is regulated by the valve / spring on the end. The pressure comes in
from the master cylinder on both the upper ports. It immediately gets sent
to the rears and when it reaches 130 psi the pressure overcomes the spring
and the valve lifts sending pressure to the fronts.
By reversing the lines all I've done is reversed which master cylinder
piston feeds the fronts and the rears.
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 Rob Mueller
Sent: Thursday, 7 January 2010 11:53 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] bad master
Bob,
Thanks! I DO NOT DOUBT what you have noted but I'm still confused!
The reason I'm confused is because of the write-up in the Maintenance Manual
X-7525. On page 5-2 paragraph entitled MASTER CYLINDER it says:
DESCRIPTION
The dual master cylinder is designed so that then front and rear brakes have
separate hydraulic systems. The hydraulic pressure developed in both systems
is equal at all times since the front piston is balance between the
hydraulic pressure in each system.
OPERATION
Fluid is directed to the wheel through two hydraulic outlets, one for the
front brakes and one for the rear brakes.
On page 5-3 the Maintenance Manual says:
METERING VALVE FUNCTION
The metering section of the combination valve operates to "hold off"
hydraulic flow (pressure) until about 130 psi has been built up in the
system before applying the front brakes. The pressure then blends to full
line pressure at approx. 400-600 psi. There is no flow restriction when the
brakes are released.
I can't picture how it does this correctly if the inlet lines are reversed
taking into consideration the information on page 5-2 as noted above.
Yeah, I know Jethro Tull got it right "thick as a brick!"
The Maintenance Manual Are there any pictures / schematics of the
combination valve that show how it works?
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 Bob de Kruyff
Sent: Thursday, 7 January 2010 8:59 AM
To: gmclist@temp.gmcnet.org
Subject: Re: [GMCnet] bad master
""IIRC the proportioning valve sends pressure to the rear cylinders first
then
to the fronts and since I've reversed them that would reverse the
sequence?""
Not Ken here, but the sequence will still be the same since the output lines
are still the same.
--
Bob de Kruyff
78 Eleganza
Chandler, AZ
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
GMCnet mailing list
List Information and Subscription Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
_______________________________________________
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] bad master [message #69842 is a reply to message #69838] |
Wed, 06 January 2010 19:53 |
Bob de Kruyff
Messages: 4260
Registered: January 2004
Location: Chandler, AZ
Karma: 1
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Senior Member |
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""By reversing the lines all I've done is reversed which master cylinder
piston feeds the fronts and the rears.""
That's it 
Bob de Kruyff
78 Eleganza
Chandler, AZ
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| Re: [GMCnet] bad master [message #70132 is a reply to message #69861] |
Fri, 08 January 2010 22:50 |
Craig Lechowicz
Messages: 541
Registered: October 2006
Location: Waterford, MI
Karma: 0
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Senior Member |
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>Comes under "one picture is worth a thousand words!" Especially to a dummy!
And as the designers like to say, One scale model is worth 1,000 pictures
Craig Lechowicz
'77 Kingsley, Waterford, MI
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