Hydro-Boost Brake Assist
Hydro-Boost brake units utilized pressure created by the power steering pump instead of engine vacuum for brake assist. This is sometimes necessary if the engine does not create enough vacuum to assist the braking system. This is true for diesel, turbo charged, and super charged engines. Some manufacturers use an engine driven vacuum pump instead of a Hydro-Boost brake system.
A Hydro-Boost unit is located between the firewall and the master cylinder just like a vacuum booster. It has hydraulic lines leading from the power steering pump and to the power steering gear much like the power steering lines incorporated into the power steering system. The unit uses a spool valve and power piston to regulate pressure and brake assist. A spring loaded or nitrogen gas charged accumulator is used in case of a power steering system failure.
The hydraulic lines and fittings should be visually inspected for leaks. The hydraulic accumulator can be located internally in the power piston or externally on the booster unit. The externally mounted accumulator is under pressure and should be tightly fastened to the unit. The power steering system and pump must be functioning properly. If this system does not provide the pressure required to operate to hydroboost unit braking performance will be affected. It may be necessary to check the power steering system for pressure and flow before proceeding.
Testing a Hydro-Boost unit is similar to diagnosing a vacuum boost unit. With the engine off, pump the pedal several times. This discharges the accumulator. Hold the pedal to the floor and maintain pressure on the pedal while starting the engine. The pedal should move slightly toward the floorboard and then move back toward your foot.
