GM Lumina/Silhouette/Transport APVs 1990-1999 Repair Guide

Master Cylinder


On newer vehicles (including those with ABS), the master cylinder is a composite design (plastic reservoir and aluminum body). It is used in a diagonally split system (one front and one diagonally opposite rear brake served by the primary piston, and opposite front and rear brakes served by the secondary piston). Left front and right rear are on the same circuit, and the right front and left rear are on the same circuit.

The functions of a standard dual master cylinder are incorporated; in addition, it has a fluid level sensor and integral proportioners. The proportioners are designed to provide better front to rear braking balance with heavy brake application.

See Figure 1

Click image to see an enlarged view

Fig. Fig. 1: Exploded view of a non-ABS master cylinder


For vehicles equipped with ABS, the entire Hydraulic Assembly must be removed along with the master cylinder. Please refer to the procedure, located under the ABS system in this section.

Brake fluid contains polyglycol ethers and polyglycols. Avoid contact with the eyes and wash your hands thoroughly after handling brake fluid. If you do get brake fluid in your eyes, flush your eyes with clean, running water for 15 minutes. If eye irritation persists, or if you have taken brake fluid internally, IMMEDIATELY seek medical assistance. If your vehicle is equipped with ABS, please refer to the Hydraulic Modulator replacement procedure

  1. Disconnect the negative battery cable.
  3. Use a turkey baster or equivalent tool to siphon the brake fluid from the master cylinder into a suitable container.

Clean, high quality brake fluid is essential to the safe and proper operation of the brake system. You should always buy the highest quality brake fluid that is available. If the brake fluid becomes contaminated, drain and flush the system, then refill the master cylinder with new fluid. Never reuse any brake fluid. Any brake fluid that is removed from the system should be discarded. Also, do not allow any brake fluid to come in contact with a painted surface; it will damage the paint.

  1. Unplug the electrical connection from the fluid sensor.
  3. Use a flare nut wrench to disconnect the hydraulic lines from the master cylinder. Plug or cap the lines to avoid contaminating the system.
  5. Unfasten the two master cylinder-to-power booster retaining nuts or bolts, then remove the master cylinder from the vehicle.

See Figures 2, 3 and 4

Click image to see an enlarged view

Fig. Fig. 2: Exploded view of the master cylinder-to-power booster attachment-vehicles without ABS