Power brake units used on passenger

Power brake units used on passenger cars are of four general types: vacuum suspended; air suspended; hydraulic booster, and electro-hydraulic booster. Most power brakes use vacuum suspended units, which contains a large vacuum-powered booster device to provide the added thrust to the typical power-brake. Pressure on the brake pedal pushes forward a rod connected to the pistons of the two master cylinders. The pistons begin forcing fluid into the front and rear brake lines. At the same time, the brake-pedal pushrod positions the vacuum-control valve so that it closes the vacuum port and seals off the forward half of the booster unit Power brakes are great when they work correctly, but what happens when they don't? Suddenly it takes a lot more effort to bring the vehicle to a halt. This may increase the distance it takes to stop the vehicle and create a potentially dangerous situation. So if a vehicle is suffering from increased pedal effort, the first thing you should check is the power brake system. VACUUM BOOSTERS Most power brake systems use a vacuum booster to assist braking. Vacuum is good for a lot of things, like sucking gas through a carburetor, opening and closing airflow doors in a vehicle's climate control system, and for providing extra muscle for braking. But to assist braking, it takes a lot of vacuum. Vacuum is the absence of atmospheric pressure. Higher the vacuum, the greater the atmospheric push to fill the void. So the push that a vacuum brake booster gives to the master cylinder is actually atmospheric air pressure. It is obvious that the vacuum booster needs two things to do its job: a good vacuum supply from the engine, and a good diaphragm. In order to replace a vacuum booster is an easy job. All one has to do is disconnect it from the brake pedal on the inside and unbolt the master cylinder. The pushrod that runs from the booster into the back of the master cylinder must have the specified amount of play, so check your service manual for the particulars. Most require a small amount of play so the master cylinder will release fully preventing brake drag. HYDRO-BOOST Though not as common as vacuum booster power brake systems, the Bendix "Hydro-Boost" system dates back to 1973. The system uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide power assist. Inside the Hydro-Boost unit, which fits between the master cylinder and brake pedal the same as a vacuum booster, is a spool valve and piston assembly. When the driver steps on the brake pedal, brake pad factory the pushrod slides forward and changes the position of the spool valve. This opens a valve port that routes power steering fluid into the cavity behind the piston to push it forward and apply the brakes. Problems with this system can be caused by spool valve or piston wear inside the Hydro-Boost unit, fluid leaks or loss of pressure (worn pump, slipping pump belt, etc.). ELECTROHYDRAULIC On vehicles with integral anti-lock brake systems where the master cylinder is part of the hydraulic control assembly, an electric pump with a nitrogen pressurized accumulator is used to provide power assist. With these systems, power assist is provided by pressure stored in the accumulator. When the driver steps on the brake pedal and the pushrod moves forward, it opens a valve inside the master cylinder that allows stored pressure from the accumulator to enter a cavity behind the piston assembly. This pushes the piston forward and applies the brakes. A pressure switch on the master cylinder monitors the stored pressure in the accumulator, and closes a switch to turn on the electric pump when pressure drops below a preset minimum. It then turns the pump off when pressure is back up to where it should be. The most important thing to keep in mind about servicing these systems is to always depressurize the accumulator before working on any part of the brake system or opening up any plumbing. The pedal needs to be pumped 30 to 40 times with the engine off (or until an increase in pedal effort is clearly felt) to bleed off all the pressure from the accumulator.