In the 90's, power brake assist was not vacuum assisted, rather it was provided by a hydraulic system known as the ATE H31 Hydroboost in BMW’s high end cars up to the 850Ci. I’ve read in places that BMW chose this for the V12 powered 850i and 750iL because of the difficulty in fitting the larger vacuum booster into the very cramped engine bay. Certainly those engine bays are tight and hard to work on but I suspect BMW were just as interested in using the performance of this unit in their flagship cars.
There are 3 main components to this system - the pressure accumulator, the booster and the regulator.
The accumulator is often known as the "Bomb," not just because of it’s shape but because of the high pressure it can contain. The bomb is filled with a nitrogen compartment that gets compressed by the hydraulic fluid. This ‘charge’ remains in the bomb even when the system is turned off. The bomb can release a large amount of pressure through the fluid very quickly when required. Fluid from the pump flows to the regulator where it is diverted to both the bomb, and the power steering box (the same pump drives both systems). You can see two electrical connectors on the regulator - these attach to pressure sensors - the top one monitors the pressure coming from the pump, and the side mounted one monitors the much higher pressure at the accumulator bomb. The driver can then be alerted to failure in either of these systems.
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The system still works even if all engine power is lost
Fluid from the accumulator travels to the input of the booster unit (top right port in picture), there is also a return line to the reservoir (top left port). Simply put, when the brake pedal is pushed in (rod on the right hand side) the hydraulic pressure assists in pushing the output rod (would emerge on left hand side of picture) which acts on the brake master cylinder input rod. One of the performance benefits of the system is that the hydraulic force is constant - it doesn’t vary according to engine conditions like a vacuum system could. A safety benefit is that the system still works even if all engine power is lost - the rule of thumb is that the system should provide 15-20 power brake assists even when the engine is off. Any less and you should suspect failure in the accumulator.
On cars 15-20 years old (or younger) the main problem with the system is leaks - the seals inside the booster can give up, leaks can occur at both pressure switches, and the accumulator itself can fail. (a leaking diaphragm allowing nitrogen to leak out and mix with the hydraulic fluid) Luckily all of these parts are still available to buy new.
About the Author: Bryan McPhail
Bryan is a longtime BMW enthusiast in Florida.
