GM Full Size Vans 1987-1997 Repair Guide

Hydro-Boost

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Some models are equipped with the Bendix Hydro-Boost system. This power brake booster obtains hydraulic pressure from the power steering pump, rather than vacuum pressure from the intake manifold, as in most gasoline engine brake booster systems.

SYSTEM CHECKS



  1. A defective Hydro-Boost cannot cause any of the following conditions:
    1. Noisy brakes
    2.  
    3. Fading pedal
    4.  
    5. Pulling brakes
    6.  

  2.  

If any of these occur, check elsewhere in the brake system.

  1. Check the fluid level in the master cylinder. It should be within 1 / 4 in. (6mm) of the top. If is isn't, add only DOT-3 or DOT-4 brake fluid until the correct level is reached.
  2.  
  3. Check the fluid level in the power steering pump. The engine should be at normal running temperature and stopped. The level should register on the pump dipstick. Add power steering fluid to bring the reservoir level up to the correct level. Low fluid level will result in both poor steering and stopping ability.
  4.  


CAUTION
The brake hydraulic system uses brake fluid only, while the power steering and Hydro-Boost systems use power steering fluid only. Don't mix the two!

  1. Check the power steering pump belt tension, and inspect all the power steering/Hydro-Boost hoses for kinks or leaks.
  2.  
  3. Check and adjust the engine idle speed, as necessary.
  4.  
  5. Check the power steering pump fluid for bubbles. If air bubbles are present in the fluid, bleed the system:
    1. Fill the power steering pump reservoir to specifications with the engine at normal operating temperature.
    2.  
    3. With the engine running, rotate the steering wheel through its normal travel 3 or 4 times, without holding the wheel against the stops.
    4.  
    5. Check the fluid level again.
    6.  

  6.  

REMOVAL & INSTALLATION



See Figures 1 through 10



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Fig. Fig. 1: Common hydraulic booster mounting



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Fig. Fig. 2: Potential leakage points on the Hydro-Boost unit



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Fig. Fig. 3: Exploded view of the Hydro-Boost unit



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Fig. Fig. 4: Use a rag to catch the fluid when disconnecting the pressure hoses



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Fig. Fig. 5: Always use a flare wrench on the fittings



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Fig. Fig. 6: Note the O-ring on the end of the fitting. Use a new one when reconnecting


CAUTION
Power steering fluid and brake fluid cannot be mixed. If brake seals contact the steering fluid or steering seals contact the brake fluid, damage will result!

  1. Apply the parking brake.
  2.  
  3. Turn the engine off and pump the brake pedal 4 or 5 times to deplete the accumulator inside the unit.
  4.  
  5. Remove the two nuts from the master cylinder, and remove the cylinder keeping the brake lines attached. Secure the master cylinder out of the way.
  6.  
  7. Remove the hydraulic lines from the booster.
  8.  
  9. Loosen the retainers and separate the booster unit from the firewall.
  10.  
  11. Disconnect the pushrod from the brake pedal and remove the unit
  12.  

To install:
  1. Engage the pushrod to the brake pedal and install the unit.
  2.  
  3. Tighten the booster mounting nuts to 22 ft. lbs. (30 Nm) on 1987-95 models and 27 ft. lbs. (36 Nm) on 1996-97 models.
  4.  
  5. Install the master cylinder.
  6.  
  7. Bleed the Hydro-Boost system.
  8.  



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Fig. Fig. 7: Loosen and remove the booster mounting nuts from inside the vehicle



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Fig. Fig. 8: Disconnect the pushrod from the pedal



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Fig. Fig. 9: Withdraw the booster assembly by pulling straight out



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Fig. Fig. 10: Once the booster is out far enough to clear the pushrod, it can be removed from the vehicle

BLEEDING



The system should be bled whenever the booster is removed and installed.

  1. Fill the power steering pump until the fluid level is at the base of the pump reservoir neck. Disconnect the battery lead from the distributor.
  2.  

Remove the electrical lead to the fuel solenoid terminal on the injection pump before cranking the engine.

  1. Jack up the front of the vehicle, turn the wheels all the way to the left, and crank the engine for a few seconds.
  2.  
  3. Check steering pump fluid level. If necessary, add fluid to the "ADD" mark on the dipstick.
  4.  
  5. Lower the car, connect the battery lead, and start the engine. Check fluid level and add fluid to the "ADD" mark, as necessary. With the engine running, turn the wheels from side to side to bleed air from the system. Make sure that the fluid level stays above the internal pump casting.
  6.  
  7. The Hydro-Boost system should now be fully bled. If the fluid is foaming after bleeding, stop the engine, let the system set for one hour, then repeat the second part of Step 4.
  8.  

The preceding procedures should be effective in removing the excess air from the system, however sometimes air may still remain trapped. When this happens the booster may make a gulping noise when the brake is applied. Lightly pumping the brake pedal with the engine running should cause this noise to disappear. After the noise stops, check the pump fluid level and add as necessary.

 
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