Do you see any rust on the caliper piston or have any reason to believe that there may be rust internally? Brake fluid is hygroscopic, which means it attracts water from the atmosphere. If the brake fluid is very old, it's possible that water has been circulating throughout the hydraulic system that operates the calipers. And if the piston can't retract from the brake disc when you release the brake (lever?), then it will stay engaged and squeal. Then you'll need either a new caliper or you'll need to rebuild this caliper, which takes some specialty equipment.

There are devices to check the moisture level of your brake fluid, but basically if it's black and nasty instead of clear and honey-colored, there's water (and rust) in there.

Good luck!

It's likely the bore of the slave cylinder rather than the piston, which will still look fine. When you push the piston back to accommodate new pads it will be going back into an area of the cylinder which hasn't seen action for some time; that's the area which is likely to be rusty.

Personally I'd simply replace the complete caliper. You'll need to bleed it afterwards.

Sounds like it's all corroded and needs to be rebuilt. The former owner probably rode it through streams a lot and water could have found it's way inside.

The swing-arm/axle bearings will probably be the next thing to go bad.

I never know how to reply again here. Hope this will work. The piston looked fine, no visible rust, also the brake fluid looked good, clear.

yes

I have a 2001 Yamaha wolverine 350. I wanted to change the rear brake pads, after removing the old ones I couldn't compress the piston back in, I had the brake fluid cap and diaphragm off, I used an old brake pad to put over the piston and then used a C clamp, it wouldn't budge, I also opened the bleeder valve. There has always been an awful squeal coming from the brakes hence changing the pads, they actually didn't look that bad and there are no groves on the fly wheel. I am wondering now if it has been seized all along, I really haven't had it out since I bought it used. The one time I did take it out the squealing got worse as the day went on. what can I do to get the piston back in, is there a tool, a trick, can I take it all apart, and if it needs a whole new caliper is it easy to install. Frustrated ! 50 year old female, I don't know too much, although I did get the front pads changed okay, and by okay I mean 2 hours for the 10 minute job they show on youtube. p.s, I cant afford a mechanic so that's not an option.