GM Cavalier/Sunfire 1995-2000 Repair Guide

Master Cylinder



See Figure 1

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Fig. Fig. 1: The master cylinder and ABS hydraulic modulator assembly

When the ABS modulator cylinder pistons are in their uppermost position, each motor has prevailing torque due to the force necessary to ensure each piston is held firmly at the top of its travel. This torque results in gear tension, or force on each gear that makes motor pack separation difficult. To avoid injury, or damage to the gears, the Gear Tension Relief Sequence briefly reverses each motor to eliminate the prevailing torque. This procedure is one of the many functions of GM's Tech 1® scan tool. Use care when using a substitute. In general, make sure the ignition switch is in the OFF position. Install the Tech 1® or equivalent with the correct chassis cartridge. Turn the ignition switch to the ON position, leaving the engine OFF. Select the proper function. The Gear Tension Relief Sequence is F5 on the Tech 1® scan tool. Note that this same scan tool is needed to bleed the system after repairs to the hydraulic system.

To perform the Gear Tension Relief Sequence, a Tech 1® tool or equivalent scan tool must be used prior to removal of the ABS modulator/master cylinder assembly.

  1. Perform gear tension release procedure using the Tech 1® scan tool or equivalent.
  3. Remove the battery.
  5. Remove the air cleaner assembly.
  7. Detach the electrical connector from fluid level switch.
  9. Detach the electrical connectors from the ABS solenoids.
  11. Detach the 3-pin and 6-pin motor pack electrical connectors from the ABS hydraulic modulator.
  13. Remove the 4 brake pipe tube nuts from master cylinder and modulator assembly. If necessary, tag for identification and placement. Place shop cloths on top of the motor pack to catch any dripping fluid. Take care not to allow brake fluid to enter the bottom of the motor pack or the electrical connectors.

When disconnecting the brake lines from the master cylinder and removing the unit, use care not to spill brake fluid on any painted surfaces or electrical connectors. Once the brake lines are disconnected from the master cylinder, plug the lines to prevent excess brake fluid loss and contamination.

  1. Remove the master cylinder mounting nuts. It may be necessary to remove the vacuum check valve from the vacuum booster to access the nut closest to the check valve.
  3. Remove the master cylinder and modulator as an assembly.
  5. Separate the modulator assembly from the master cylinder assembly. Use the following procedure.
    1. Turn the assembly upside down so the flat gear cover is facing up.
    3. Remove the 6 gear cover attaching screws. This should allow access to the motor pack screws.
    5. Remove the 4 motor pack screws and separate the motor pack from the ABS modulator assembly.
    7. Remove the 2 Torx head through bolts that retain the master cylinder to the motor pack assembly. Use care not to loose or damage the 2 small transfer tubes that connect between the lower part of the master cylinder assembly and the hydraulic modulator assembly. Watch for O-rings seals at both the transfer tubes