10 bolt posi rear

I'm going to be picking up a 10 bolt posi rear, 3.73's, with disc brakes, out of an '85 IROC.

My question is about the master cylinder. From '84 thru '88 I find a seperate listing for a 2 wheel disc and a 4 wheel disc car. From '88 on however, the only master listed is the same as the one listed for 2 wheel disc starting in '84. My car is a '91, which only has the single listing for a master cylinder, but I am assuming that I will need one that is listed for 4 wheel disc. Any thoughts?

p.s. Does anyone know where I can get ahold of the proportioning valve I'll need besides the dealer, or a junk yard?

get the part number that matches the rear you are putting in the car. the rear is from the 85, then get the master cylender for the 85 four wheel disk.

as far as where to get your parts, ACDelco, go to the dealer, your brakes are not something i would trust to some unknown aftermarket company.