Master Cylinder


When a master cylinder is defective, take it apart and use what you know about master cylinder operation to locate the source of the problem. Master cylinders can be purchased as new or rebuilt units. Rebuild kits are available for use when the cylinder bore is not damaged by corrosion.

A master cylinder is rebuilt to replace leaking seals or gaskets. If a more serious problem exists, the master cylinder should be replaced.

  • To remove a master cylinder, disconnect the brake lines at the master cylinder. Install plugs in the brake lines and master cylinder to prevent dirt from entering.
  • Remove the nuts that attach the master cylinder to the fire wall power brake unit, and remove the cylinder.
  • Remove the cover and seal. Drain the master cylinder and carefully mount it in a vise. Remove the piston assembly and seals according to the manufacturer's instructions. New pistons, pushrods, and seals are usually included in rebuilding kits.
  • Clean master cylinder parts only with brake fluid, brake cleaning solvent, or alcohol. Do not use a solvent containing mineral oil, such as gasoline. Mineral oil is very harmful to rubber seals.
  • Inspect the master cylinder. Damage, cracks, porous leaks, and worn piston bores mean the master cylinder must be replaced.
  • Check very carefully for pitting or roughness in the bore. If any are present, the cylinder must be replaced.
  • Reassemble, install, and bleed the master cylinder according to the manufacturer's directions.
Brake fluid will remove paint. Always use fender covers to protect the vehicle's finish and take extra care not to spill brake fluid.

Bench Bleeding

Before installing a master cylinder, it can be filled with fluid and bled of air.

  • Pumping the apply rod forces air and fluid out of the piston chambers. New, airless fluid enters the master cylinder through the reservior.
  • A variation of the process is to hold your fingers over the fluid outlets while releasing the pedal rod. This prevents air from being sucked back into the piston chamber.
  • When the pedal rod is applied, fluid and air can escape into a drain pan.
  • The process is repeated until all air is expelled from the mastery cylinder.
  • Quick take-up master cylinders have a step bore in which one piston is larger than the other. The large piston provides low pressure but high volume to take up clearance between the linings and rotor with less pedal travel.
  • Feel the bottom of the master cylinder.
  • If the casting is larger near the rear, it is a quick take-up clyinder.
  • These are bench bled with a suction device. Without that, use a large Phillips screwdriver or wooden dowel to slowly depress the rear piston 3/4 of the way into the bore.
  • Do not bottom out the piston.