Classic Motorcycle Build

Remove Stuck Brake Piston

Article by Mark Trotta

Under normal circumstances, a brake caliper rebuild involves removing, disassembling, cleaning, and reassembling the caliper with new seals. But sometimes there's the unexpected surprise of a stuck piston.

remove stuck piston from brake caliper

Corrosion built up between the piston and it's bore is usually what makes calipers stick. This often happens to vehicles that sit for a long time without use. Two popular methods for removing a stuck brake piston are compressed air and soaking.

Tools Needed

A garage air compressor with a blow gun attachment may be all you need to remove a stuck brake piston. A blow gun with a rubber tip will give best results. They seal better, plus it'll prevent you from accidentally scratching something.

air blow gun mounted in vise

With the bleeder screw closed, shoot about 50-60 psi of compressed air into the brake hose hole. In most cases, this is sufficient enough to blow out a stuck piston.

stuck brake piston removal

CAUTION: When using compressed air to remove a stuck caliper piston, Wrap an old towel around the caliper and hold it with your other hand, or the piston will fly across the garage (yes, I've done this).

You may get lucky and have the piston pop out on the first try. That was not the case with this 1981 Sportster front caliper.

unstick a stuck brake piston

Even with 100 psi of air, the piston would not come out.

Soaking The Caliper

With the caliper propped upwards in a pan, squirt some brake parts cleaner into the brake hose entry. I've also used WD40 with good results.

soak brake caliper to remove stuck piston

I let the caliper sit for two hours. Soaking overnight is best, but sometimes time is a bigger factor.

After soaking, I could move the piston a fraction of inch, but it was still not ready to come out.

Next, with a fresh razor blade, I cut the old rubber boot around the piston, so I could spray brake parts cleaner inside and around it. More soaking followed.

stuck brake piston removal

The piston finally came out, and the caliper rebuild resumed.

Brake Caliper Cleaning and Inspection

caliper rebuild

All metal brake parts can be cleaned with brake parts cleaner and shop towels. Do not use brake parts cleaner on rubber, as it will eat them.

remove stuck brake piston

With a flat-blade screwdriver, lightly scrape any corrosion inside the grooves where the caliper O-ring will seat. Follow with fine sandpaper or a scuff pad.

Harley brake caliper rebuild

After cleaning, blow dry with compressed air.

Brake Caliper Piston

Clean the piston and inspect it for nicks and burrs.

caliper piston

The piston can be cleaned with brake parts cleaner and a scuff pad, then wet-sand with fine (600 or 800) sandpaper. If you have any deep vertical scratches or pitted marks that won't come out, the piston should be replaced.

Brake Caliper Reassembly

This caliper is now ready for reassembly.

brake caliper parts after cleaning


Brake Fluid Types

Most disc-equipped Harley motorcycles require DOT 5 brake fluid. An easy way to find out which brake fluid type you have is to put a few drops of it in a styrofoam coffee cup. DOT 3 and DOT 4 brake fluids are glycol-based, and will dissolve the cup, DOT 5 (silicone) will not.

motorcycle master cylinder 1976 Sportster

Do not mix brake fluid types.


Related Article:

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Read: Rebuild Harley Dual Disc Calipers

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Read: Harley Pie-Slice Caliper Rebuild

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Read: Best Thread Sealant For Brake Fittings