Argh! I'm pretty frustrated -- three of the four corners went well on my brake job today. but the last has got me stumped. I can't get the piston to retract into the caliper. It seemed fine when I was driving, so I can't imagine it's seized (and the brake pads have worn evenly).
Any thoughts, tips or advice?
E36 M3 (the car, not the filter at play).
The good news is that a reman caliper is less than a $100. The bad news is that is three days away, minimum.
Have you tried a bigger hammer? Or possibly opening the brake reservoir cap? Then again I'm sure a torch could help as well.
Andrew
open the bleeder screw while you're pushing the piston back. don't send that skanked fluid upstream, especially on an ABS-equipped car.
but re. the failure mode, there are lots of things it could be, but if you didn't have symptoms like pulling one direction, one badly worn (or un-worn) rotor, or oddly worn pads in that caliper, IDK why you wouldn't be able to push in the piston.
The piston may be cocked in the calibre bore. Put the most worn pads on that corner, put the caliper back over the rotor and press the brake pedal. The piston should pop out enough and be square in the bore. You may need something thinner than the rotor like a thin board. Obviously you arent driving the car with this. 
Press the piston back in carefully so as not to cock it in the bore.
The down side is you may have scored the bore or piston. If so you will need to replace the caliper.
Is that one of the pistons that has to screw back in?
What kind of car?
Edit: Oh, I see that it is a BMW M3
Get a real C-clamp and put your purse down 
I once spent over an hour trying to get the pistons to retract in the 08 rabbit, then i figured out they need to be rotated back in with the special tool. quick trip to horror freight, $15 and five minutes later i was done.. those sneaky Germans... 
Since these are floating calipers... you already have it off the car to do the pads. At this point I'd just remove the hose from the caliper and take it to the bench. Put a piece of wood in the bore and use the air compressor to push the piston out. 20psi will do it. 100 will do it dramatically (watch your fingers). Clean it and make sure there isn't a nick in the wall of the piston or corrosion that wont make a good seal with the o-ring.
If there isn't or its just surface stuff from dirty brake fluid, clean it gently with emory cloth and then wash it. Autozone sells a rebuild kit for about $4 that includes the dust boot and rubber seal. Its easy to do once you get the hang of it but buy 2 just in case. The dust boot can give you fits on your first try. If there is deep corrosion or big ding, order the caliper. They are cheap enough that its not worth welding and turning it to fix (unless you happen to own the lathe).
Good advice all. I bit the bullet and ordered the remanufactured caliper. I looked carefully at it, and didn't like the looks of it, especially for track use. Safety first, safety always :)
Smart move.
Do you have to return the core or can you disassemble it and tell us how the story ends?
Woody wrote: Smart move. Do you have to return the core or can you disassemble it and tell us how the story ends?
There's a sixty-dollar core charge -- so I will return it. But I might attempt to disassemble it first, to see what I can learn.
Had this happen to me once. Turned it it was a bad flex line. It had collapsed inside. The fluid would only go one way. Pushing the fluid back actually closed up the line inside.
E36 M3 is not the rotate back in type of caliper. It just sounds stuck to me. What can happen is that the rubber boot rips, then the bore gets rusted behind the caliper so when you try to push it back in it gets stuck on the oxidation. Sounds like you need a new caliper. For any high performance car, and especially a track car I highly suggest replacing calipers together. The other side has been doing the exact same thing for the exact same time and miles...how much longer can it have? And is it worth it to not find out until your new rotors and pads are trashed?
Bad flex lines / brake hoses can collapse inside and act like one-way valves. Had done dozens of brake jobs over the years, and never saw this until a couple of years ago on a 2000 Chrysler minivan. The caliper was effectively stuck in such a way that it would stop the wheel when brakes applied, but the brake pads were dragging on the rotor, and of course the piston could not be pushed back in with a clamp. Replacing the hose completely cured the issue.
easy to check a bad hose causing the problem, open the bleeder screw. piston retracts, bad hose. Piston does not retract, it is stuck.
With the caliper apart, the piston should move freely in the caliper bore without the seal. The seal is what retracts the piston.
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