atv rear brake?

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.

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