Master cylinder trouble

[kwlskid]

Hello All,

I’m doing some maintenance on my 1972 GMC stepside ‘heavy 1/2 ton’, and am having trouble finding the correct master cylinder. The original was leaking and the 2 replacements that have been tried don’t work right way too much braking on the front and almost none on the rear.
The truck was my grandfathers and as far as I know and can tell it came with front disc brakes, a 12-bolt rear end, and leaf springs. I don’t know if any of that info. changes things but with the confusion on the correct part I’m guessing it may.

Can anyone tell me what part number(s) would be correct to replace the master cylinder, and booster (might as well it’s almost 50 yrs old). And can anyone tell me anything about the differences between the reg 1/2 tons and the ‘heavy 1/2 tons’

Thanks!

[tutone]

Before you throw the baby out with the bathwater, you say you have replaced th M/C twice with a "like kind" replacement? If you are experiencing heavy braking on the front and very little on the rear, I would take a look at my rubber flex lines on the front discs. If you have any deteriation inside the lines, they could be holding pressure on the front brakes and robbing pressure, so to speak, from the rear. Next, I might take a look at the proportioning valve. If it sticks in the right position, there is a chance that it might cause you issues. If all this fails, you would then start at the booster/master cyl. Here is a doorman number. Should be available at most parts stores.

[LockDoc]

-
If the braking on the rear isn't what you think it should be also check the rear flex hose. They are prone to deteriorating on the inside and restricting flow to the rear brakes.

LockDoc

[Steeveedee]

Was the master a direct fit, without having to use any adapters between it and the brake lines? It could be that you got a 3/4T master. Also, it is possible that the shuttle valve moved when you bled the brakes. This will stop all flow to the rear brakes, if that is the way the valve moved.

[body bolt]

Last Saturday I installed this one on my '71 C-10. Bled it on the truck and it 's working fine.

[jocko]

Do you have 5 lug or 8 lug wheels? (there's really no such animal as a Heavy Half in 72)

If 5 lug, you need a C10 MC (like the one tutone posted)
If 8 lug, you need a C20 MC

The above MCs are quite different as their reservoirs route to different ends of the truck - which could possibly explain your unbalanced braking performance (as could improper rear drum brake adjustment or the shuttle valve as Steeveedee mentioned or hoses as Leon mentioned).

A C10 MC's forward reservoir provides fluid to the front brakes (and brake lines on this circuit should be 3/16").
A C20 MC's forward reservoir provides fluid to the rear brakes (and brake lines on this circuit should be 1/4")

(regardless of C10 or C20, remember that FRONT disk brake lines are 3/16" -and- REAR drum brake lines are 1/4" - from the factory)

Does your forward most reservoir's brake lines go thru the distribution block and then to the front - or rear - brakes?

Hope this helps - any pics will help us help you, incl SPID.