Hydroboost Brake Assist
Hydroboost brake units use power steering pressure 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, turbocharged, and supercharged engines. Some manufacturer's use an engine driven vacuum pump to operate the vacuum booster instead of a installing a hydroboost brake system.
A hydroboost 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 to the power steering gear, much like the lines incorporated into the power steering system. It uses a spool valve and power piston to regulate pressure. 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 doesn't provide enough pressure to operate to hydroboost unit, braking performance will be drastically affected. It may be necessary to check the power steering system for pressure and flow before proceeding.
Testing a hydroboost unit is similar to diagnosing a vacuum booster. 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.
* It's important to remember that the hydroboost brake system is assisted by the power steering system.