Likely the piston is cocked in the bore and rusty to boot from being run on too thin a rotor (why or why do people do this?????).

Try tapping the piston toward the bottom of the caliper while applying force INWARD -- if it's sidways, this may straighten it out enough to get it out.

Never use more than 15 psi of air pressure using a blower tip. Higher pressure won't likely move a stuck piston, but if it does come out, the high pressure air expands explosively, and the piston is heavy enough to crack your head (that is, kill you outright). Not funny. At LEAST put a board in there so the piston cannot come free, but it's best to use air only to push a moving one free.

Put the other piston back in if you havent yet, install the brake line but leave the caliper hanging, and use a C clamp to hold the good piston in place. Bleed the air out and STAND on the pedal -- likely you can get the piston to move a bit. If the pedal sinks, keep pumping until it pops out (no exposion here!). If you cant get it to move, get another caliper, that one is shot.

Do not use any piston that is not perfect below the seal, it won't retract properly and/or the pits will eat the seal and it will leak. The pistons are chrome plated, and should NEVER be cleaned with anything more abrasive than cloth and brake parts cleaner. Pits ABOVE the seal are OK, although they can cause trouble.

You can remove rust above the piston seal in the bore with crocus cloth, very fine sandpaper. or Scotchbrite, but you MUST NOT touch the area below the seal. Clearance is very tight, embedded abrasives (there will be some no matter what you do) will remove the clearance and the piston will seize, or you will remove too much material and the piston will go sideways and stick...

If in doubt, get a new one.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Well, I've reworked 3 calipers now and am have fair success. The real question is 'how long will they last?' But the price is right and I'm learning some things along the way.

Thanks for all the advice. Much of it was helpful in getting the stuck pistons out. I used wet-800 sandpaper very lightly and it seemed to do the trick.

Don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed

Just replaced a RR caliper on my 87 TD wagon since I couldn't get one stuck piston out on the old caliper. I was using brake pressure with no luck. Well, when I put on the new caliper, I couldn't bleed the brakes and the new pistons weren't pushing out against the new pads. Read a post about bleeding the brakes and realized that while I thought the fluid reservoir was full, the back part of the reservoir which is for the rear brakes, was empty. I had to overfill the reservoir so that fluid would spill over the internal divider. After doing this, I was able to easily breed the rear brakes and activate the new pistons. Makes me wonder if I had tried this on the old caliper, maybe the piston would have popped out. Will probably try it and if it works I will rebuild it and sell it as a rebuilt caliper.
Don't know if you are working on rear calipers, but thanks to the shop forum, I was able to get my new brake parts working perfectly.

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83 300TD wagon
87 300TD wagon
89 BMW 325IX

Do NOT rebuild and sell the caliper.
Liability would destroy you for the rest of your life, if anything happened.

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https://whunter.carrd.co/

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Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
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1985 300SD
1983 300D
2003 Volvo V70

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Thanks....you're right it is not worth the risk!

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83 300TD wagon
87 300TD wagon
89 BMW 325IX

The above comment relates to my experience. If it corroded and stuck it will do it again and it will not take as long the next time. Brakes are too critical to gamble with for the paltry return. They are inexpensive, why take a risk?

When I was young and poor and couldn't afford parts to fix vehicles I would rebuild anything, not any more.

Now that I have said that I will reverse and say that if the car is a daily driver the rebuild will more likely have a long service life. If you rebuild the caliper and the car sits most of the time, expect it to stick again soon.

Alternators, starters and carburetors I rebuild. The thinking is that if you take a proven component with worn parts and put in new parts then you have a proven component with little or no wear. When you buy a rebuild you don't know where it has been. Actually if I never see another carburetor I will feel no loss. I guess they are still good for spraying the flowers and such.