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Hydroboost Braking SystemsPosted 2/12/1999By David Tidaback
If you are dealing with a vehicle that uses a hydroboost braking system found on certain BMW, Jaguar and Audi models, such as 1980-1992 BMW 7-series, 1983-1992 BMW 533 and 535 models, 1982-1989 BMW 6-series, 1988-1989 Jaguar XJ-6, or 1984-1992 Audi 5000, 100 and 200 models, this might interest you. Consider the following sequence of events: A 1988 BMW 735 is brought in with a set of brake-related complaints. When the car is first started, the brake pedal is high and hard, and the car wants to creep forward in gear unless a really heavy foot is used on the pedal. After driving a block or so, the brakes feel normal and seem to operate fine. While waiting at a traffic light with the car in gear, the pedal suddenly sinks, often with a jerking, stepwise motion. A quick pump on the pedal brings it back to normal height, but it is hard again. Rolling down the street, the brakes feel fine once more, but a quick stab of the pedal to avoid a car pulling into the lane ahead causes a "Brake Pressure" warning light in the instrument cluster to come on briefly. Symptoms of these sorts may come and go with changing driving conditions. The car owner is not sure when the symptoms began to appear, but he knows that they have suddenly become much worse. Initial inspection of the brake system does not show anything obviously wrong. The fluid is topped up and clear; no leaks are visible, and pads and rotors are all in good condition. A tech who is unfamiliar with this car's braking system may readily come to the conclusion that the master cylinder has worn seals that are allowing internal leakage, causing the pedal to drop. This does nothing to explain the intermittent high and hard pedal, but it seems a good place to start. Unfortunately, if the master cylinder is replaced, the problems that the car was brought in for will remain. What the technician needs to know is that this car has a brake booster system unlike the one found in most other cars, and that malfunctions of this system may cause all of the symptoms mentioned above. The car has no vacuum booster; instead, it has a power steering pump (or in the case of the Jaguar, a separate engine-driven pump) that also supplies pressure for braking assist. The brakes need to have boost available for emergency situations even when the engine has died while driving, so there needs to be a reservoir to hold pressure. This reservoir is called the brake pressure accumulator. The accumulator is a hollow, thick-walled steel ball or cylinder with a flexible diaphragm dividing the interior into two chambers. One chamber is completely sealed and pressurized with nitrogen to approximately 1,100 psi. The other has an inlet for power steering fluid that is pumped in until the nitrogen pressure in the accumulator is over 2,000 psi and the fluid fills a large part of the accumulator's interior volume. The accumulator is mounted on a valve block that controls the direction of flow - from the pump into the accumulator and out of the accumulator to the booster. When the brake pedal is pressed, a valve is opened in the booster that admits pressure from the accumulator to the back side of the booster piston, pushing it forward to operate the master cylinder. Due to the high pressures involved, the actual volume flowing from the accumulator to the booster is very small. This means that a good accumulator can store enough reserve pressure for several emergency stops, even with the engine not running. With a bad accumulator the story is very different. If the accumulator loses its internal pressure, or a part of it, it also loses the reserve capacity for emergency stops. If it loses enough pressure, eventually it will not be able to supply the booster for even one stop without falling below the minimum working pressure. This turns on the warning light and causes a loss of boost, resulting in a hard brake pedal and increased braking effort. When the internal pressure of the accumulator approaches about 525 psi minimum, an alternately hard and soft pedal may be encountered, as the power steering pump will be able to keep the pressure above the minimum until the brakes have been operated for a certain time, then a loss of boost will occur as it falls below the minimum. This is when the stepwise sinking of the pedal mentioned above will be noticed. Also due to very high pressures, special pressure gauges and hoses are needed to test this system, and most shops don't have them unless a large proportion of their shop volume consists of cars with hydroboost systems. A simple and accurate test for a failed or failing accumulator can be made without special equipment. If the car is run at idle for a couple of minutes without the brake pedal being pressed, then the engine is shut off, a new accumulator should allow at least 12 pumps of the pedal (and possibly as many as 20) before it comes up high and hard, indicating loss of boost. A high, hard pedal on the first pump, or even after four to five pumps, indicates an accumulator that has failed or is failing and should be replaced. Replacement of the brake pressure accumulator is a straightforward procedure that involves unscrewing the accumulator from the valve block, or in some cases, replacing the accumulator and valve block as a unit. Before loosening the accumulator or fittings, be sure to pump the brake pedal at least 20 times to bleed off any residual pressure, otherwise you may be wearing power steering fluid while finishing the job. Always be sure to follow the manufacturer's recommendation regarding steering fluid specifications. Top up the fluid in the reservoir only after running the engine. Pump the brake pedal until it is hard to bleed as much fluid as possible back into the reservoir; otherwise it might overflow during emergency braking conditions. No bleeding of the system is necessary. A little knowledge of system testing and operation can save a lot of time and frustration in brake repairs. If you have any questions, we're as close as your phone!
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Copyright (c) 1996-2009. Automotive Service Association®. All rights reserved.
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