Hello all i have an 89 iroc z lo3 with the open diff drum rear end i just bought an 85 rear end with disc brakes and am going to swap it in was told I'll need a prop valve and master cylinder from a couple people and then a couple told me i can just change the spring in my prop valave that i have now and that my master is the same please help me understand this thanks in advance for any help

 

Did ya try a search? 85 discs are Delco Moraine iron calipers, aka crap. Don't be surprised if you make that swap and the brakes work worse than the stock drums.

The DM discs use a different master and prop valve calibration. In 89 GM finally gave up on the house brand brakes and switched to PBR calipers that actually work. The 89-up PBR rear disc uses the same master as the drum cars. They still use a different prop valve. Changing the spring is a cheat to adjust the brake balance. It's not a proper recalibration, it's a short cut to what may or may not be better.

The PBR discs are a nice upgrade from drums. The DM calipers are more of a pain than drums, and they dont work as well.

 

Why not just stick to drums? I stayed with my drums when I changed rear ends but I did pickup some aluminum drums. I don't do circle track racing so they work fine for me Lol.

 

I have to echo about the early third gen rear calipers not being much. I swapped my 84 from drum to the rear disc. Used new ACDelco reman calipers. They are quite pityful compared to the PBR set ups used in leter 3rd gens.

 

I don't claim to be an expert, but from everything I've read over the years, the early rear disc brakes suck and aren't worth swapping to from drums, and the later rear discs are quite a bit better...