School me on master cylinders

[C15_Sierra]

My '80 has manual brakes, and I want to convert them to power brakes to better complement the 5.3

I got a used '84ish booster and master cylinder that has all sorts of rust/nasty crud in the forward (front?) compartment. Rear is relatively clean.

First, is the master salvageable? Can I take it apart/clean it, or is already FUBAR? Is it recommended to try and repair?

Second, if I go the "just replace it" route, LMC lists a whole bunch of master cyls for trucks with power brakes. I'm guessing my plain jane Sierra came with the smallest brakes (SPID doesn't mention any upgraded brakes), at 11", does that refer to drum diameter? Should I match the master cylinder to the booster (like I said, '84ish but I can't be certain, I didn't pull the part from the truck) or should I match it to my '80 chassis? What are the differences?

Thanks!

Also..side note - there's a connector that has been cut off (probably by me when I was doing the motor swap) at the proportioning valve. What is that originally for?