1. Brake fluid reservoir
2. Master cylinder
4. Actuator unit
5. Pressure side
6. Return side
Although not as common as a conventional brake system fitted with a vacuum booster, many vehicles are now equipped with hydraulically assisted boosters for the brakes. The system uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide the power assistance required in a conventional system. This application is particularly suitable to vehicles with diesel engines as a separate vacuum source does not have to be provided for the system to operate.
Because the system uses fluid pressure from the existing power steering pump the booster uses the pressure from the fluid that is always circulating through it, as the source of pressure that applies against the master cylinder actuating piston.
The hydraulic pressure generated by the power steering pump is stored in an accumulator, which is then routed to the master cylinder by the hydraulic booster unit when the brake pedal is applied.
When applied the booster can generate pressures of between 1,200 to 2,000 psi or 8273 to13789 Kpa to the brake calipers. The systems are generally available with or without a matched the master cylinder. The systems that have an included master cylinder have a reservoir as part of the assembly.
As a safety measure part of the system includes a component to assist in the maintenance of system pressure known as an "accumulator." Some are nitrogen pressurized while others are spring loaded depending on the application. In the case where pressure is lost (such as when the engine stalls or power steering pump drive belt breaks) the systems accumulator is designed to store sufficient pressure to provide for three full-power applications. If this is insufficient, the system then resorts to manual brakes.
Operation problems can be caused by a number of things such as leaks inside the booster unit, by a worn power steering pump, slipping or broken pump drive belt, or hose connections.
A simple way to test the system is to pump the brakes five or six times with the engine off to discharge the accumulator. Press down hard on the pedal and then start the engine. Like a vacuum booster, you should feel the pedal fall slightly when the engine starts, then rise again.
The leak-down in relation to the capacity of the accumulator can be checked by pumping the brakes several times whilst the engine is running and then shutting the engine down. The vehicle should then be left for about an hour, and the brakes applied without starting the engine. In an efficient and operational system it should be possible to get 2 or 3 soft brake applications before it takes more effort to push the pedal.