Typical power-assist vehicle brake systems use engine vacuum to power the pedal assist, providing more hydraulic force in the brake lines than a purely mechanical system can easily produce without extreme driver effort. The Hydroboost system uses hydraulic pressure supplied by the power steering pump instead.
Using the power steering pump provides several advantages. Primarily, no longer having to rely on engine vacuum. This is extremely important in diesel vehicles since the engine does not produce enough vacuum to activate a brake booster. The power steering pump can also provide more power than a vacuum system, which can power bigger brakes, or just normal brakes better.
Like a vacuum operated system, the engine must be running to get repeated use out of the system. Most hydroboost systems feature a reserve canistor to power the brakes at least once in the event of engine failure, allowing the vehicle to stop in an emergency situation. This is another advantage over most vacuum systems, as these do not have an emergency reserve.
Hydroboost systems are in some ways more compact, not having a large vacuum canistor at the booster. But they do have hydraulic lines and sometimes coolers that require checking and maintenance. A leak in this hydraulic system affects not only the brakes but the power steering as well, as the systems are linked having the same pump. The power steering pump provides more than enough pressure to supply the power steering alone, so the pump itself is usually nothing special. In most cases for ease of installation, a pump with a second return port is used, although not always necessary depending on hydraulic routing design.
The brake system can be designed to use a dedicated pump, although this is not necessary. Most systems use the same pump for brake and steering assist because of efficiency and simplicity. In more rare circumstances, an electrical powered pump is used to provide a more extensive emergency backup. This is more common on larger vehicles that require lots of power to stop, but are not equipped with air brakes.
The system is self-purging, as fluid is circulated continuously throughout. No purging of air should be necessary after maintenance.
The hydroboost unit operates off a rod plunger from the brake pedal, just like standard vacuum brakes. This rod controls the internal valving of the system to guide hydraulic pressure to the brakes, or a bypass in varying amounts.
Because the system cannot operate properly without pressure from the pump, if the engine were to die brake function and boost pressure would be lost immediately. For this reason a reserve canistor is almost always provided, which is pressurized by system hydraulic pressure and stored in the event system pressure drops or is lost. The canistor is usually calibrated in size to allow at least one full application of the brake pedal to bring the vehicle to a stop. This canistor is basically a hydraulic accumulator.