How to Overhaul Master Cylinders

The master cylinder is the pump that creates the pressure to be transferred to the individual wheels to stop a vehicle. The brake system is under tremendous pressure when the brake pedal is pushed. That pressure is amplified greatly by a vacuum or electric brake booster, better known as power brakes. It is vital that the integrity of the brake system be strong enough to handle the extreme pressure. Any pedal fade or cylinder leaks found in the master cylinder must be addressed by rebuilding or replacing the cylinder.

Instructions

Put on safety glasses, uncap the master cylinder and pour the brake fluid out of the reservoir.

Clamp the master cylinder in a vise. Remove the push-rod dust boot and the internal snap ring from the open end of the cylinder bore, using snap ring pliers.

Remove the double cup piston plunger assembly from the master cylinder and place the internal parts on a clean shop rag in the order in which they were removed.

Spray and clean the entire master cylinder inside and out, using brake cleaner. Dry up any remaining brake cleaner and fluid using a shop rag or compressed air, if available.

Inspect the empty cylinder bore for scratches. Spray the cylinder bore with penetrating oil and run a brake hone, chucked in a 3/8 drill motor, in and out of the cylinder bore to clean it up and remove any scratches.

Spray and re-clean the entire master cylinder, including the cylinder bore and fluid reservoir.

Spread the contents of the master cylinder repair kit onto a clean dry shop rag. The kit should include at minimum a new double cup piston, push-rod dust boot and snap ring.

Pour a small amount of new brake fluid into the cylinder bore.

Insert the new master cylinder piston and plunger kit in the same order as the old internal parts were removed and are displayed on the shop rag. Consult and follow the instructions for any changes or component improvements in the new kit.

Push the piston plunger into the cylinder while installing a new internal snap ring into the groove at the end of the cylinder bore.

Fill the master cylinder with clean brake fluid while the master cylinder is clamped upright in the vise.

Screw the two short brake lines, one into each chamber of the master cylinder line openings.

Bend the two lines, without kinking them, so the open end is submerged in brake fluid in the master cylinder reservoir. Pump the piston plunger in the cylinder bore with a screwdriver until no air bubbles are seen coming out of the two brake lines.

Remove the two short brake lines, reinstall the master cylinder cap and install the dust boot over the open cylinder bore end. The master cylinder is ready to be installed on the vehicle.