UPDATE:

Yesterday, I disassembled the caliper and used a C-clamp to free the caliper pistons, but now they’re stuck inside the caliper. What should I do now to prevent them from seizing again while they’re inside?

 

Replace the caliper is the best path. Brake fluid absorbs water and some has found its way down to the caliper. It corrodes the caliper bore and/or piston, and then it will seize up. This will no get better and there is no way to get the corrosion and crud out by flushing.

A caliper rebuild sometimes lasts well but sometimes does not. So a replacement is the worry-free way to fix this. Getting the pistons out of the caliper is can be tricky, and it easiest with compressed air to gradually work each piston out.... but not totally free so that the air can work on the others piston(s). On small calipers, that gets tricky.

This is a common problem for cars, trucks, sleds, ATV's, etc..... It is good to find and fix this properly as stuck calipers have been reported to cause fires in sleds.....!!!

 

I managed to use the air compressor to remove the piston and clean it. I put it back on the caliper with new fluid, but I think I’m not bleeding it correctly or the piston isn’t retracting properly the pad stays against the disc. The pistons aren’t stuck though, when I push them a bit, they move back in. Maybe I’m just not bleeding it the right way.

 

The 'o-ring' seal against the piston is square in shape. It distorts with a twisting when the piston pushes out and this twist in the ring relaxes and pulls the piston back when the brake is released. This often does not fully pull the pads away from the rotor but takes all the pressure off of the disc. Without being there, it is impossible for us to see if this is just light dragging of the pads or there is still real pressure on the pads due to piston not retracting properly.

If there is still pressure on the pads against the disc when the lever is released and the pistons move freely if you push them back by hand, then it could be that the master cylinder is clogged and not allowing the fluid to move back with the 'untwisting' force of the caliper's piston ring(s). Or there still may be some corrosion in the caliper bore that is still there. If you have to use the c-clamps to move the piston(s) back then they are not moving freely like they should; they should move back with hand force..

When the caliper piston was pulled out, was the caliper bore(s) examined to see if there was any corrosion there?

Another possibility is that the brake line had deteriorated internally and a piece of the inner lining is being pulled back into the line when the fluid tries to move back into the master cylinder. this will occur with older rubber brake lines. (I have not heard of it happening with teflon brake lines.)

 

No corrosion in the cylinder, it was brand new, grey in color, untouched, no residue or rust at all. The bore of the piston and the cylinder was a bit dirty, but once cleaned, it looked like new. The outer piston looked old and dirty, but I cleaned it as well. When I push it by hand, it’s not hard to move it back in. Of course, I can’t remove it by hand when it’s in the cylinder, I have to use the air compressor for that. It moves smoothly, like new. I also applied some brake fluid on the sides of the piston and cylinder before pushing the piston back in, and it went in easily by hand. It’s like I just don’t know how to bleed it correctly. When I release the brake lever, the brake pad should come off the disc.

 

Is it correct that there was no corrosion in the caliper bore/Just want to be reading the above correctly.

Bleeding issues would rarely or never produce a stuck piston in the caliper. The problem would be the opposite.....you would have a hard time applying the brakes. Sounds like the piston is moving freely since things look clean and you can move the piston by hand.

The sounds like a master cylinder sticking or a brake line deteriorating inside. The latter is not at all uncommon when rubber brake lines get to the 15-20 year old range; a bit of the lining cracks and folds back and blocks the return of the fluid to the master cylinder and can absolutely hold the brakes on.

 

I've dealt with this problem lots. I've found two causes.

Cause #1:

The issue is in the master cylinder. There are two orifices at the bottom of the reservoir.

When the fluid gets old and absorbs water, it makes stuff corrode, and for some reason this tiny orifice likes to corrode over with brown rust. It's very small, so once it's rusted over it can withstand incredible pressure. It doesn't let the fluid return to the reservoir, so it keeps the piston jammed against the rotor.

In the picture below it's the one with the red arrow pointing up.

Open up the reservoir and soak the fluid up with a paper towel. Use a needle or pick or awl to pick at the corrosion in that orifice. DO NOT move the brake lever with the pick in the orifice or you will damage the seals.

Once your brakes are working good, flush your fluid through the system yearly.

Cause #2

One time I got a chunk of something (rust?) and it was stuck in the rubber line. It would act as a check valve, and stick the piston. I was able to blow air both ways through the line freely, but with fluid it would stop the flow. Had to buy a new line.


also, this was the result of that orifice clogging:
at the time we figured he left the park brake on (hence the thread title).

https://www.dootalk.com/threads/dont-forget-to-release-your-park-brake-roflmao.1637597/?post_id=24494676#post-24494676

 

Good post above....

Cause #2

One time I got a chunk of something (rust?) and it was stuck in the rubber line. It would act as a check valve, and stick the piston. I was able to blow air both ways through the line freely, but with fluid it would stop the flow. Had to buy a new line.

also, this was the result of that orifice clogging:
at the time we figured he left the park brake on (hence the thread title).

FWIW, this is exactly what happens when the inner liner in the brake line deteriorates and a chunk breaks out. It often become an instant check valve and it can hold the brakes on hard. May be hard to believe if you have not dealt with it before.....

Would enjoy hearing what the OP found in this case.