What to Do If the Disc Brake Piston Does Not Fully Retract?
2 Answers
First, determine whether the issue lies with the piston or the sealing ring. 1. Retraction Principle: The disc brake piston retracts using the negative pressure generated by the master cylinder. As for the sealing ring, it relies on tension for sealing, but the minimal gap is primarily ensured by the precision of the machining between the pump body and the piston. If the gap is too large, the sealing will fail. 2. Piston Rust: The cause is rust formation on the piston due to water ingress or corrosion under natural conditions, preventing normal retraction. You can remove the piston from the slave cylinder by 5-10mm and spray rust remover or use fine sandpaper wrapped around the piston, rotating it several times. 3. Sealing Ring Aging: If the rectangular rubber sealing ring is aged, swollen—or if the piston is smooth but difficult to press back—consider issues like aging, swelling, or deformation of the sealing ring. The rectangular rubber sealing ring must be replaced.
I've dealt with incomplete brake caliper piston retraction many times. Common causes are either piston seizure due to rust or lack of lubrication on guide pins. First use the caliper adjuster to push the piston back, then after removing the caliper, focus on cleaning rust and grease deposits from the piston surface - especially impurities in the grooves must be thoroughly sprayed with brake cleaner. If the dust boot is aged or torn, replace it immediately to prevent recurrence from dust ingress. Always use special green high-temperature grease (regular grease fails when heated). Before reassembly, repeatedly test piston movement and check brake fluid condition - replace if too dirty and bleed the system. Remember never force components during disassembly to avoid damaging precision parts.
