GM Service Manual Online
For 1990-2009 cars only

Disassemble Procedure

Important: The master cylinder reservoir cover and reservoir diaphragm can be inspected and/or serviced without removing the master cylinder from the vehicle.


    Object Number: 481544  Size: MH
  1. Wipe the reservoir cover (1, 2) clean before removing.
  2. Remove the master cylinder. Refer to Master Cylinder Replacement .
  3. Remove the master cylinder reservoir cover (1) and master cylinder reservoir diaphragm (2).
  4. Caution: Brake fluid may irritate eyes and skin. In case of contact, take the following actions:

       • Eye contact--rinse thoroughly with water.
       • Skin contact--wash with soap and water.
       • If ingested--consult a physician immediately.

    Notice: Brake fluid will damage electrical connections and painted surfaces. Use shop cloths, suitable containers, and fender covers to prevent brake fluid from contacting these areas. Always re-seal and wipe off brake fluid containers to prevent spills.

  5. If the master cylinder is to be completely removed and overhauled, empty the fluid from the reservoir (3).
  6. Inspect the master cylinder reservoir cover (1) and diaphragm (2) for cuts, cracks, nicks and deformation. Replace any damaged parts.

  7. Object Number: 482967  Size: SH
  8. Remove the fluid level switch (3).

  9. Object Number: 481544  Size: MH
  10. Remove the piston snap ring (12) while depressing the primary piston (11).
  11. Do not damage the primary piston, master cylinder bore, or the piston retainer groove.

  12. Remove the primary piston and the primary piston spring.
  13. In order to remove the parts, perform the following steps:

    8.1. Plug all of the outlet ports, except the upper outlet port.
    8.2. Apply low-pressure non-lubricated compressed air into the upper outlet port at the blind end of the bore.
  14. Remove the secondary seal (8), the primary seal (10) and the spring retainer (7) from the secondary piston (9).
  15. Clean all of the parts in clean, denatured alcohol.
  16. Dry the parts using non-lubricated compressed air.
  17. Inspect the master cylinder bore for scoring or corrosion. Replace the master cylinder if scoring or corrosion is present.

Assemble Procedure


    Object Number: 481544  Size: MH
  1. Lubricate the rubber parts with clean brake fluid in order to ease assembly.
  2. Install the lubricated secondary seal (8), primary seal (10) and the spring retainer (7) onto the secondary piston (9).
  3. Install the secondary piston spring (6) and the secondary piston assembly (9) into the cylinder bore. Lubricate the parts with clean brake fluid in order to ease assembly,
  4. Install the lubricated primary piston (11) into the cylinder bore.
  5. Install the piston retainer (12) while depressing the primary piston (11).
  6. Install the fluid level switch (14).
  7. Install the master cylinder reservoir diaphragm (2) into the reservoir cover (1).
  8. Install the reservoir cover and diaphragm onto the master cylinder reservoir (3).
  9. Install the master cylinder. Refer to Master Cylinder Replacement .