Subaru Coupes/Sedans/Wagons 1985-1996 Repair Guide

Power Brake Booster

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The power brake booster uses engine manifold vacuum against a diaphragm to assist in the application of the brakes. The vacuum is regulated to be proportional to the pressure placed on the pedal.


CAUTION
Properly disarm the air bag on vehicles equipped with the SRS system. Failure to do so can cause serious injury. The procedure can be found in (see Chassis Electrical ) under the heading "Supplemental Restraint System".

If brake performance is questionable and the booster unit suspect, conduct the following tests.

AIR TIGHTNESS TEST



  1. Apply hand brake and start engine.
  2.  
  3. Run the engine for one or two minutes, then turn it off.
  4.  
  5. Apply brakes several times using the same force as in normal braking. The pedal stroke should be greatest on the first application and become smaller with each additional stroke. If no change occurs in the pedal height while it is applied the power brake unit could be faulty.
  6.  

OPERATION CHECK



  1. With the engine off. Apply the brakes several times using normal pedal pressure, make sure the pedal height does not vary on each stroke.
  2.  
  3. With the brakes applied, start the engine.
  4.  
  5. When the engine starts the brake pedal should move slightly toward the floor. If no change in the pedal height occurs the power brake unit could be faulty.
  6.  

INSPECTION



Inspect the vacuum hose and check valve periodically, the hose for cracking or brittleness. The check valve (engine running and brakes applied) for air leaks. Replace hose or valve if necessary. Sometimes a stuck check valve can act like a bad power booster. If this is suspected, replace the check valve.

Rebuilding a power brake booster or doing a complete pressure test requires special gauges and tools. It is just not practical for the car owner to attempt servicing the unit except to remove and replace it.

REMOVAL & INSTALLATION



Justy
  1. Disconnect the negative battery cable.
  2.  
  3. Remove the master cylinder.
  4.  
  5. Disconnect the vacuum line from the power booster.
  6.  
  7. From under the dash board remove the snap ring from the booster pushrod.
  8.  
  9. Remove the pushrod clevis pin.
  10.  
  11. Remove the power booster mounting nuts.
  12.  
  13. Remove the power booster from under the hood.
  14.  

To install:
  1. Install the power booster on the firewall.
  2.  
  3. From under the dash board install the mounting nuts and tighten to 13 ft. lbs. (18 Nm).
  4.  
  5. Connect the pushrod to the brake pedal and install the clevis pin and snapring.
  6.  
  7. Install the master cylinder on the power booster.
  8.  
  9. Connect the vacuum line to the booster.
  10.  
  11. Bleed the brake system.
  12.  
  13. Connect the negative battery cable.
  14.  

Coupe, Sedan, XT, Loyale, Wagon and Brat

See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Brake booster pushrod and connecting hardware

  1. Disconnect the negative battery cable.
  2.  
  3. From under the hood perform the following:
    1. Disconnect the electrical harness from the fluid level indicator.
    2.  
    3. Disconnect and cap the brake lines from the master cylinder.
    4.  
    5. Remove the master cylinder mounting nuts.
    6.  
    7. Disconnect the vacuum hose from the booster.
    8.  

  4.  
  5. From under the dashboard perform the following steps:
    1. Remove the snap pin and clevis pin securing the booster pushrod to the brake pedal.
    2.  
    3. Remove the four power booster mounting nuts.
    4.  

  6.  
  7. Remove the power booster from under the hood. When removing the booster be careful not to damage the brake lines.
  8.  

To install:
  1. Install the brake booster on the firewall.
  2.  
  3. From under the dashboard install the four mounting nuts and tighten to 13 ft. lbs. (18 Nm).
  4.  
  5. Connect the booster pushrod to the brake pedal and install the clevis pin and snap pin.
  6.  
  7. Connect the vacuum line to the booster.
  8.  
  9. Install the master cylinder and tighten the mounting nuts to 10 ft. lbs. (14 Nm).
  10.  
  11. Connect the brake lines to the master cylinder and tighten the fittings to 11 ft. lbs. (16 Nm).
  12.  
  13. Connect the electrical connector to the brake fluid level sensor.
  14.  
  15. Bleed the brake system.
  16.  
  17. Connect the negative battery cable.
  18.  

Impreza and Legacy
  1. Remove the master cylinder.
  2.  
  3. Disconnect the vacuum hose from the brake booster.
  4.  
  5. Working under the instrument panel, remove the cotter pin and clevis pin from the operating rod bracket. Remove the brake booster mounting nuts.
  6.  
  7. Remove the brake booster from the engine compartment.
  8.  

To install:
  1. Position the power booster in the engine compartment.
  2.  
  3. Install the mounting nuts. Tighten the nuts to 9-17 ft. lbs. (13-23 Nm).
  4.  
  5. Install the clevis pin with a new cotter pin.
  6.  
  7. Adjust the brake booster operating rod.
  8.  
  9. Connect the vacuum line to the booster.
  10.  
  11. Install the master cylinder.
  12.  

SVX

See Figures 2 and 3

Click image to see an enlarged view

Fig. Fig. 2: Cutaway view of the SVX's brake booster



Click image to see an enlarged view

Fig. Fig. 3: Remove the brake booster retainer nuts from inside the vehicle

  1. Disconnect the negative battery cable.
  2.  
  3. Raise and safely support the vehicle on jackstands.
  4.  
  5. Disconnect the sway bar from the frame.
  6.  
  7. Drain about 1.5 qts. of transmission fluid from the transmission.
  8.  
  9. Remove the bolts securing the upper and lower sides of the transmission dip stick tube.
  10.  
  11. Remove the cruise control actuator, if equipped.
  12.  
  13. Disconnect the positive battery cable from the starter.
  14.  
  15. Discharge the A/C system using an approved recycling/recovery machine.
  16.  
  17. Remove the low side A/C pipe.
  18.  
  19. Disconnect the vacuum line from the brake booster.
  20.  
  21. Remove the master cylinder from the power booster.
  22.  
  23. From under the dash, remove the snap ring and clevis pin from the brake booster pushrod at the brake pedal.
  24.  
  25. Remove the four nuts mounting the power booster to the firewall.
  26.  
  27. Remove the power booster.
  28.  

To install:
  1. Install the power booster on the firewall and install the four mounting nuts. Tighten the mounting nuts to 9-13 ft. lbs. (13-18 Nm).
  2.  
  3. Connect the power booster pushrod to the brake pedal and install the clevis pin and snapring.
  4.  
  5. Install the master cylinder on the power booster.
  6.  
  7. Connect the vacuum line to the power booster.
  8.  
  9. Install the low side A/C pipe.
  10.  
  11. Recharge the A/C system using an approved recycling/recovery machine.
  12.  
  13. Connect the positive battery cable to the starter.
  14.  
  15. Install the cruise control actuator.
  16.  
  17. Install the dip stick tube mounting bolts.
  18.  
  19. Top off the transmission fluid.
  20.  
  21. Connect the sway bar to the subframe.
  22.  
  23. Lower the vehicle.
  24.  
  25. Connect the negative battery cable.
  26.  

ADJUSTMENT



Legacy and Impreza
  1. Turn the engine off.
  2.  
  3. Disconnect the vacuum line from the brake booster.
  4.  
  5. Loosen the operating rod locknut and adjust the rod until there is 0.04-0.12 inch (1-3mm) of play in the brake pedal.
  6.  
  7. Tighten the locknut.
  8.  

 
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