GM Astro/Safari 1985-1996 Repair Guide

Master Cylinder

Print

REMOVAL & INSTALLATION



See Figures 1, 2, 3, 4, 5, 6 and 7



Click image to see an enlarged view

Fig. Fig. 1: All master cylinders are held with two nuts, regardless of the booster type



Click image to see an enlarged view

Fig. Fig. 2: Remove as much of the used fluid as possible from the reservoirs



Click image to see an enlarged view

Fig. Fig. 3: Use only flare wrenches on the line fittings or the fitting might get damaged



Click image to see an enlarged view

Fig. Fig. 4: Use a rag to catch any brake fluid that spills, otherwise the fluid could cause paint damage



Click image to see an enlarged view

Fig. Fig. 5: Be careful when removing the master cylinder nuts, they are close to the positive battery terminal



Click image to see an enlarged view

Fig. Fig. 6: Remove the master cylinder and check for fluid in the booster



Click image to see an enlarged view

Fig. Fig. 7: Use fresh fluid when reinstalling the master cylinder

The master cylinder removal and installation procedures are basically the same for all brake systems. Always use flare nut wrenches to remove the hydraulic brake line. Damage to the fitting nut may occur if this procedure is not followed.

  1. Disconnect the negative battery cable. Apply the parking brakes or block the wheels.
  2.  
  3. Using a siphon, remove and discard some of the brake fluid from the master cylinder reservoirs.
  4.  
  5. Disconnect and plug the hydraulic lines from the master cylinder using flare nut wrenches only.
  6.  
  7. If equipped with a manual brake system, disconnect the pushrod from the brake pedal.
  8.  
  9. Remove the master cylinder-to-bracket (manual) or vacuum booster (power) nuts, then separate the combination valve/bracket from the master cylinder.
  10.  
  11. Remove the master cylinder, the gasket and the rubber boot from the vehicle.
  12.  

To install:
  1. Bench bleed the master cylinder and install the cylinder onto the vehicle. Torque the master cylinder mounting nuts to 28 ft. lbs. (38 Nm).
  2.  
  3. Connect the hydraulic lines and torque to 15 ft. lbs. (20 Nm).
  4.  
  5. Refill the master cylinder with clean brake fluid, bleed the brake system and check the brake pedal travel.
  6.  

If equipped with manual brakes, be sure to reconnect the pushrod to the brake pedal.

OVERHAUL



See Figures 8, 9 and 10



Click image to see an enlarged view

Fig. Fig. 8: Remove the reservoir from the master cylinder by gently prying it off



Click image to see an enlarged view

Fig. Fig. 9: Exploded view of the master cylinder



Click image to see an enlarged view

Fig. Fig. 10: Press and rock the master cylinder to seat the reservoir in the grommets

  1. Remove the master cylinder from the vehicle.
  2.  
  3. At the rear of the master cylinder, depress the primary piston and remove the lock ring.
  4.  
  5. Block the rear outlet hole on the master cylinder. Using compressed air, gently direct it into the front outlet hole to remove the primary and secondary pistons from the master cylinder. If compressed air is not available, use a hooked wire to pull out the secondary piston.
  6.  


CAUTION
If using compressed air to remove the pistons from the master cylinder, DO NOT stand in front of the pistons, for too much air will cause the pistons to be fired from the master cylinder, causing bodily harm.

  1. From the secondary piston, remove the spring retainer and the seals.
  2.  
  3. Using the mounting flange (ear) on the master cylinder, clamp it into a vise.
  4.  
  5. Using a medium pry bar, pry the reservoirs from the master cylinder. Remove the reservoir grommets.
  6.  

DO NOT attempt to remove the quick take-up valve from the master cylinder body; the valve is not serviceable separately.

  1. Using denatured alcohol, clean and blow dry all of the master cylinder parts.
  2.  
  3. Inspect the master cylinder bore for corrosion or scratches; if damaged, replace the master cylinder with a new one.
  4.  

To install:
  1. Use new reservoir grommets (lubricated with brake lube) and press them into the master cylinder body. Install new seals onto the primary and secondary pistons.
  2.  
  3. Position the reservoirs on flat, hard surfaces (block of wood), then press the master cylinder onto the reservoirs, using a rocking motion.
  4.  
  5. Using heavy duty brake fluid, meeting DOT 3 specifications, lubricate the primary and secondary pistons, then install them into the master cylinder. While depressing the primary piston, install the lock ring.
  6.  
  7. Install new diaphragms onto the reservoir covers.
  8.  
  9. Install the master cylinder onto the vehicle and torque the mounting bolts to 28 ft. lbs. (38 Nm). Bleed the system.
  10.  

 
label.common.footer.alt.autozoneLogo