This is on my wife's 1987 IROC with stock 4 wheel disc brakes.

I put a new master cylinder and rear calipers on tonight and cannot get the pads to seat down onto the rotor. I bench bled the master cylinder before putting it in. I have bled and re-bled both front and rear calipers and the rear pads will not seat on the rotors unless you pump the brake pedal 3-4 times and hold it. But as soon as you let off the brake pedal the calipers suck back in, leaving about 1/4" or more gap between the caliper piston and the brake pad. I used the search function and tried everything I could find. I tried various combinations of bleeding the front and rear brakes, tried using the parking brake to set the pads, etc. None of it made any difference. I have no idea how to fix this and get those pads to seat on or very near to the rotor as they should be. I've bled a lot of brakes over the years and have never seen a set of calipers do this. The front brakes are seated properly, this is only a problem with the rear pads seating.

 

Update:

I spent all morning working on this looking through my GM shop manual and online and realized it has pretty much everything to do with the stupid adjuster mechanism inside the caliper. The problem seems to be with the total lack of fine adjustment with the e-brake lever (attached to the caliper) and the caliper piston.

I removed the e-brake lever on the back of the caliper and used a 14mm wrench to manually adjust the piston so that the brake pads were barely brushing the rotor. Then I put the e-brake lever back on. The driver side fit right, but the passenger side would not. Due to the hex shape of the nut it mounts to I was left without any ability to fine-tune the adjustment and it was either too tight (too much pressure on pads and rotor) or way too loose. I think I solved this by cutting a notch into the e-brake lever to allow a small amount more adjustment and slack to keep the pads from being pressed too tight.

Re-bled and the brakes work okay, but I still have to figure out if I can adjust the e-brake cables. For now it is not hooked to the calipers because that added a little too much tension on the pads / rotor. I may have to loosen the cable adjuster some more.

 

That's how I used to adjust mine to get through inspection. Best solution is a set of gen3 gen4 or c4 vette pbr calipers and discs

 

I think my other option would have been to use a spanner wrench on the piston face to manually turn the piston out to the point the brake pads would lightly brush the rotor. I would have had to make small adjustments until the caliper and shoes barely slid on over the rotor. I started to do this, but realized the little tab on the brake shoe would not line up with the d-shape notch on the face of the piston. I didn't want to grind off the tab, so I adjusted the piston from the other end where the e-brake lever attaches. Looking at it now it would probably be easier and better to just grind off that tab and adjust them that way to avoid any adjustment issues with the e-brake cables.
The previous person who did these brakes actually didn't line up the tabs on the brake shoes anyway. Not sure how much that affected my brakes, but they at least still sat in the right position. The little clip helps with that too I guess.