GM Chevy Mid-Size Cars 1964-1988 Repair Guide

Hydro-Boost

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See Figure 1

Hydro-Boost differs from conventional power booster brake systems, in that it operates from power steering pump fluid pressure rather than intake manifold vacuum. The Hydro-Boost unit contains a spool valve with an open center which controls the strength of the pump pressure when braking occurs. A lever assembly controls the valve's position. A boost piston provides the force necessary to operate the conventional master cylinder on the front of the booster.

A reserve of at least two assisted brake applications is supplied by an accumulator which is spring loaded on earlier and pneumatic on later models. The accumulator is an integral part of the Hydro-Boost II unit. The brakes can be applied manually if the reserve system is depleted.

All system checks, tests and troubleshooting procedure are the same for the two systems.



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Fig. Fig. 1: Exploded view of the Hydro-Boost system mounting and installation

  1. Turn the engine off and pump the brake pedal 4 or 5 times to deplete the accumulator.
  2.  
  3. Remove the nuts from the master cylinder, then move the master cylinder away from the booster, with brake lines still attached.
  4.  
  5. Remove the hydraulic lines from the booster. Plug or cap the lines to prevent system contamination or excessive fluid leakage.
  6.  
  7. Remove the retainer and washer at the brake pedal.
  8.  
  9. Remove the nuts retaining the booster to the cowl, then remove the booster assembly from the vehicle.
  10.  

To install:
  1. Position the booster in the vehicle and loosely install the retaining nuts to hold it in place.
  2.  
  3. Remove the caps or plugs and connect the hydraulic lines
  4.  
  5. Reconnect the retainer and washer at the brake pedal, then tighten the booster-to-cowl retaining nuts to 15 ft. lbs. (20 Nm).
  6.  
  7. Reposition and secure the master cylinder by tightening the nuts to 22 ft. lbs. (30 Nm).
  8.  
  9. Bleed the power steering and hydro-booster system. Do not attempt to move the vehicle until and firm brake pedal is obtained.
  10.