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?