Caliper pistons don't move freely, possible cause ?

[Avenger]

I took the car out of storage on April 1st, it had been sitting for 5 months. I immediately noticed there was something wrong with the brakes. I removed both front wheels and calipers. Brake pads were almost non existent on the passenger side but still in good condition on the driver side.

I tried moving the pistons back and forth with my hands but none of em moved. I also tried using a C-clamp to push 'em in. Some moved, some didn't. My father said it should be possible to move them by hand (obviously not working against pressure). Considering 2 of 'em didn't even move with a C-clamp, I was a little concerned. I then removed the calipers and pumped the brake pedal to take the pistons out. I obviously had to use a few C-clamps to limit the travel of those that were easier to push to avoid spreading the oil. I inspected the pistons and cylinders and don't really know what to think. Two pistons have been stamped at the factory (on the face where the piston meets the brake pad). Problem is, the hit made them out of round and chipped part of the material (only on the part of the piston that is always sticking out of the cylinder, maybe it's some kind of stopper). Nothing I ever did with this car could create that kind of damage, if it's indeed damage...

Do you guys have any idea about what could be wrong ? I'm totally lost here...

 

[n0v8or]

I've had some similar experiences, with two identified causes:
(1) Not changing brake pads often enough. I now never let them wear past 50%. As the pads become thinner, the piston protrudes more out of the bore to compensate. The outer dust boots are notoriousy poor seals, even when greased regularly, and allow contaminants to corrode and pit the outer polished surface of the piston. It then jams in the bore when you try to push it in.
(2) Not changing brake fluid often enough (at least every 24 months). Brake fluid absorbs moisture, and moisture corrodes the piston and bore, particularly when the brakes are not used for months while the car is stored at cold temperatures.

The only solution I found is new calipers.