If your 70-72 Skylark/GS has power disk brakes, it will have a proportional (prop) valve. If U add disk brakes to the rear, Do u keep the current prop valve or remove it or replace current valve with an adjustable prop valve.
Your mileage may vary but I swapped from 1970 front disc/rear drum to new master cylinder, rear discs and NO proportioning valve. I bought an adjustable one but never installed it as it stops a lot better than the disc/drum did with one. Prior to rear discs, I could lock the rear drums pretty easily in a panic stop. Now, I get the slightest squeal from the rear in a panic stop. I'm using the Right Stuff rear discs with different rotors because the included rotors were too shallow and the calipers rubbed the factory Buick chrome wheels. This rotor is about 1/2" taller and moved the caliper out of the way. Here's where it rubbed with the witness marks from the rotor shipped with the kit.
