| Re: [GMCnet] bad master [message #69752] |
Wed, 06 January 2010 00:17  |
James Moore
Messages: 71
Registered: January 2004
Karma:
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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
> _______________________________________________
> GMCnet mailing list
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