1994 GMC C1500 brake pad brake in

It sounds like you had brake fluid contaminated with a petroleum product. The fact you found the pistons retracting too much is a great observation that most people would miss. There should never be any fluid pressure remaining in the hydraulic system for disc brakes. That will cause them to drag and seriously overheat. Rear drum brakes have a residual pressure check valve in the port in the master cylinder to maintain about ten pounds of fluid pressure. That prevents air from sneaking past the lip seals in the wheel cylinders when barometric pressure changes, and, along with the internal springs, it keeps those seals from falling over and leaking. The brake shoe return springs are much more than strong enough to overcome that ten pounds of fluid pressure.

In a caliper, the only thing you have to retract the piston is the square-cut seal. That seal is stuck in the bore of the caliper, and the inner circumference sticks to the piston. When the brakes are applied, that seal sticks and bends as the piston moves out. The piston is retracted when you release the brakes by returning to its normal shape. As it does, it pulls the piston in with it just a little. The pads will still be touching the rotor but there won't be any pressure on them.

When any hint of petroleum product gets in the brake fluid, all rubber parts that contact that fluid will become mushy and grow bigger. The most common symptom is the brakes will not release and will seriously overheat due to the lip seals in the master cylinder growing past the return ports and trapping the pressurized fluid. The only proper fix for that is to discard all the parts containing rubber parts, flush and dry the steel lines, then replace all those rubber parts. That includes flexible hoses, master cylinder, calipers, wheel cylinders, combination valve, and most trucks and minivans have a height-sensing proportioning valve in the rear. That gets to be a real expensive repair, but if the vehicle also has an anti-lock brake hydraulic controller, it may be not worth repair.

When do-it-yourselfers are made aware of the results of contaminated fluid they often try to cut corners by just replacing the master cylinder and brake fluid. That will usually solve the problem for a week or two, but that contamination is in all the rubber parts, and it is going to leach back out and recontaminate any new parts. That's why the only correct fix is to replace all the rubber parts at the same time.

It sounds like you have mushy square-cut seals in the calipers due to that contamination. Common causes of that are when people repack front wheel bearings with axle grease, wipe their hands on a rag, then rebuild a caliper or use their fingertips to pop the rubber bladder seal back into the cap on the master cylinder reservoir. That residue on their fingers will do it. Most mechanics wash their hands with soap and water before handling rubber brake parts. Another somewhat common cause in shops is when an inexperienced or untrained mechanic wipes engine oil out of a funnel, then uses it to fill a brake "bleeder ball". The residue left in that funnel will contaminate all of that brake fluid and all of the cars it goes into.

There's another cause of the pistons retracting too far, but I'm pretty sure it doesn't apply here. That is loose wheel bearings. That will allow a rotor to wobble too much and push the piston back into the caliper too far. You have to push the brake pedal further than normal to run that piston back out. The clue there is the pedal will feel fine when the vehicle is standing still but it will go down too far after driving a little or hitting big bumps in the road.

I should have thought this through more or at least qualified my thinking. If the spongy pedal started right after you replaced the rotors, fluid contamination is likely not the cause. New pads and / or new rotors require you to push the pistons back into the calipers. That is where a lot of problems originate. I mentioned that square-cut seal that sticks to the piston and bends, then straightens out and pulls the piston back with it a little. As pads and rotors wear down and the piston moves out further, the seal reaches the point where it can't bend any further, then the piston slides out through it. That is how all calipers self-adjust.

Two things can happen to impede that. The most common is when dirt or moisture sneak behind the dust boot and build up on the exposed part of the piston. The dirt will form a ring and the moisture will get under the chrome plating, (if you have steel pistons), and lift it causing rust pits. Neither of those will cause a problem until you have to push the piston back in to make room for the new pads or rotors. That dirt or rust pits stick on the square-cut seal making it real hard to push the piston in. Some do-it-yourselfers use a c-clamp to get that piston to go in, but you should be able to easily pry it in with a flat-blade screwdriver and light hand pressure before you unbolt the caliper from its mount. If you NEED a c-clamp to get the piston to move, give up and replace the two calipers. We used to rebuild them at every brake job to avoid this, but today professionally-rebuilt calipers cost a real lot less than they did 25 years ago.

The other thing that is not an issue here is sediment can form in the caliper, behind the piston. That can get in the way so the piston won't go in far enough, even with that c-clamp, to fit over the new pads or rotor.

I don't know why this would start with installing only new rotors, but a lot of vehicles use "low-drag" calipers for better fuel mileage. GM was one of the first manufacturers to do this, and they used a special master cylinder design. The difference in caliper design was very insignificant, but it did result in the pistons retracting more than normal, which is what you found. To make up for the low brake pedal resulting from having to move more brake fluid to move the pistons out further during each brake application, they used a "step bore" master cylinder design, also called the "quick take-up" design. The primary piston, (closest to the power booster), is larger than the secondary piston. It moves more fluid to the front brakes to quickly get those pistons out to the pads, then you start to build normal pedal pressure. Without that master cylinder design, you will have a low pedal, but then it will be firm, not mushy.

I also didn't ask if you have anti-lock brakes. If you do, nothing will happen unless you allow the master cylinder to run empty during bleeding. At that point, getting the air bled out of the hydraulic controller may require the use of a scanner to open some valves. Without doing that, any trapped air may never work its way out since those valves only open briefly during an anti-lock stop. That air will cause a soft brake pedal.

The first thing that came to mind was when you mentioned the brakes start to grab with the pedal still high, but it can be pushed further, is the rear shoes are turned around. The shoe toward the front is the one with the shorter lining. It's only job is to grab the drum and try to rotate with it. That makes it push on the bottom of the longer rear shoe which does the stopping. If the two shoes are turned around, the front shoe applies the rear shoe which has a shorter, and less-effective lining, so a lot more pedal effort will be needed.

The next thing is if the rear shoes are new and the drums have been machined, they will have different diameters. The shoes will only contact in a small patch in the center so they won't do much. That will gradually improve until one day it occurs to you the brakes are good.

Dandy. We typically lube the six "lands", or raised spots the shoes ride on, but that's to prevent chirping when they release and wearing grooves into those areas. I'm happy to hear you solved the problem.