Here is a tech tip on how to extract seized brake caliper pistons.
When rebuilding the brake calipers on older cars, it can be very difficult to
get a seized piston out of the caliper, especially without damaging the piston
or bore. Mechanics sometimes use air pressure to free the piston, but this can
be dangerous because the piston can eject with a lot of force and speed. A
much safer and very effective solution is to use a common grease gun. After
removing the caliper, screw a standard grease fitting into the brake line hole.
Make sure that the bleed valve is closed, then attach your grease gun to the
fitting and start pumping. Even the toughest pistons will pop out. I always
wrap a rag around the whole caliper beforehand, as this helps to keep any small
bits of grease from flying out. The high viscosity of the grease causes the
piston to eject very slowly.
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