Wrong master cylinder?

[87Skier]

Looking some info...

I thought that these came with either of two or three different brake setups. I thought that they had a smaller diameter thicker booster, a larger diameter thinner booster, and a manual setup. I also thought that dependent on what booster you had is what master cylinder you had.

I have a large diameter thin brake booster, but I believe that I am running the wrong master cylinder. I replaced it about a year ago, when I redid my entire brake system before I started driving it. I bought a new one at a trusted parts store, and it was identical to the one that came off there.

The problem I am having is that the rear brakes feel like they are the only ones working. If I'm on the highway and hit the brakes hard, the rear brakes lockup, the truck gets squirrely (even when the rear don't lockup) and the brake pedal is very hard. I'm 99% sure that I have a drum brake master cylinder and not a front disc master cylinder.

Can anyone confirm this?

[bamachem]

That looks like the same master cylinder and proportioning valve I have on my 72 blazer and it performs in a similar manner. The driver rear will lock up pretty quick on you if you have to hit the brakes for anything other than a "normal" stop.

[Jurassic-1]

Could the proportioning valve be off center inside? I purchased a small tool from Brothers which is said to aid in keeping the prop valve centered when bleeding.

[87Skier]

It's a possibility, but I feel like it has to be something more related to the master cylinder itself because of the very hard brake pedal.

I'm thinking if I swap mine out for a master cylinder that has a very small and very large reservoir it will apply the front with a lot less brake pedal force. I was hoping someone had this style on theirs and could confirm this.

[Jurassic-1]

Is that master cylinder equal size on both reservoirs ? If it is it could be like the Corvette Master Cylinder for 4 wheel disc which would be the wrong one. The hard pedal could also be a failing brake booster.

[pound41]

changing the brake booster fixed mine

[chadcampbell00]

I had this exact problem with mine. I finally bypassed my factory distrubution valve on the side of the master cylinder and put an adjustable proportioning valve in the line going to the rear brakes allowing me to decrease pressure to the rear. It stops 100xs better now. My plan is to get a new distrubition valve and put it back to normal but i haven't gotten around to that yet.