Hydraulic master cylinders (m/c) are a crucial part of a hydraulic brake system. They mount the reservoir that holds a surplus of fluid and house the piston, which, when actuated by hand (or foot) pressure either directly or augmented by a vacuum booter, supplies hydraulic pressure to the brake calipers and wheel cylinders. Hydraulic brake fluid that is contaminated with dirt or other hard particulates can destroy the piston seals and score the cylinder, leading to brake fade and failure. The rubber piston seals also may be damaged by using the wrong fluid and even old age.
Wipe the m/c down with a clean shop rag to remove any dirt and debris from the outside. Remove the m/c cap and discard the old fluid. Remove the brake lines from the m/c with a flare-nut wrench and capture any escaping fluid with a clean rag. Remove the bolts or nuts holding the m/c in place and pull it from the vehicle.
Look into the open end of the m/c and locate the retaining (snap) ring. Carefully remove the snap ring and discard it. Snap rings are intended for one-time usage and should never be reused. Spring pressure will cause the piston to protrude from the m/c. Pull the piston from the cylinder and inspect the cylinder walls for pitting or scoring. Damaged cylinders should be discarded and replaced. Wash the cylinder in fresh fluid to remove any debris or dirt. Inspect the bleed-back hole in the reservoir and insure that it is clear of obstructions.
Lubricate the new piston with fresh fluid and insert it into the m/c. Push it in and hold it against the spring pressure and install the new snap ring. Reinstall the m/c onto the vehicle and install the m/c bleeders. Lead the bleeder tubes into the reservoir and add fresh fluid. Actuate the brake system until all the air is purged from the m/c. Remove the bleeders and reinstall the brake lines. Torque them snugly with the flare wrench.
Top off the reservoir with fresh fluid and actuate the brakes. Observe the bleed-back hole and insure that there is a small geyser of fluid issuing from it just as the cylinder begins to move. This will indicate a properly working system and prevent a hydro-lock from not allowing the brakes to retract and disengage at the calipers. Bleed the brakes as per the manufacturer's recommendations and top the reservoir off at the indicated "full cold" mark on the reservoir.
