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.
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.
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.
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.
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.
Even with 100 psi of air, the piston would not come out.
Soaking The Caliper
The caliper was then propped upwards, and brake parts cleaner was shot into the brake hose entry. I've also used WD40 with good results.
After an hour of soaking and several 100 psi blasts of air, the piston would still not come out.
Once again, I filled the caliper with brake parts cleaner. This time I let it sit for two hours. Soaking overnight is best, but sometimes time is a bigger factor.
After the second session of soaking and more 100 psi blasts of air, I could feel the piston move out a fraction of inch, so I pushed it back in and tried working it in and out a few times. 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.
The piston finally came out, and the caliper rebuild resumed.
Brake Caliper Cleaning and Inspection
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.
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.
After cleaning, blow dry with compressed air.
Brake Caliper Piston
Clean the piston and inspect it for nicks and burrs.
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 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.
Do not mix brake fluid types.