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.
Saturday, January 11th, 2014 AT 4:54 PM