Subaru ff-1/1300/1400/1600/1800/Brat 1970-1984 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.

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



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Fig. Fig. 1 Common power booster and master cylinder mounting

TESTING



Air Tightness
  1. Apply handbrake 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.
  6.  
  7. 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.
  8.  

Operational Check
  1. With the engine OFF .
  2.  
  3. Apply the brakes several times using normal pedal pressure, make sure the pedal height does not vary on each stroke.
  4.  
  5. With the brakes applied, start the engine.
  6.  
  7. When the engine starts the brake pedal should move slightly toward the floor.
  8.  
  9. If no change in the pedal height occurs the power brake unit could be faulty.
  10.  

Component 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



  1. Remove the master cylinder.
  2.  
  3. Disconnect the vacuum hose.
  4.  
  5. Disconnect the brake pedal from the power booster push rod by removing the clevis pin.
  6.  
  7. Remove the four nuts that mount the booster to the firewall.
  8.  
  9. Remove the booster.
  10.  
  11. Installation is the reverse of removal.
  12.  

 
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