Brake boosters are quite simple in operation. They may also be somewhat mysterious to the average do-it-yourselfer. The vacuum operated brake booster works much today as it did sixty-years ago. A vacuum brake booster consist of a chamber, divided by a diaphragm(s). The chamber is normally steel and the diaphragm is normally some form of rubber. Vacuum, usually supplied by the engine and atmospheric pressure combine to provide the assistance during braking.
When the brakes are not being applied, a two-way valve applies vacuum to both sides of the diaphragm. Equal vacuum on both sides, balance each other and there is no effect. In this state the booster is said to be at rest.
When the pedal is applied, the two-way valve moves. In this position vacuum is maintained on the front chamber, while atmospheric pressure is allowed to enter the rear. The pressure of the atmosphere rushes in, pushing the diaphragm forward and with it a push rod. This push rod applies the brakes, through the master cylinder, attached to the front of the booster.
When the pedal is released, the two way valve reverts to the at-rest position and atmospheric pressure is blocked. Vacuum is again applied to the rear of the diaphragm and with the help of a spring, the push rod retracts.
The system is robust and inherently failure resistant. Should the valve, diaphragm or vacuum source fail, the push rod is mechanically connected to the brake pedal. The system simply reverts to manual brakes.
The most common failures are lack of assistance, noise or poor pedal return after release. Many times problems are mis-diagnosed as a bad booster and many boosters are needlessly replaced. A lack of vacuum is a more common failure. Collapsed and cracked hoses are most common. Before replacing a booster, vacuum source must always be tested.
A vacuum gauge is used to check for adequate vacuum. Vacuum at the booster should be around 16-20 inches. Lower vacuum can result from a restricted source and sometimes an engine running problem. For instance a plugged catalytic converter can lower engine vacuum and result in poor booster performance.
A leaking booster can also cause an engine running problem. Leaks in the booster provide a vacuum leak to the engine. One simple test for leakage, is to turn off the engine and press the brake pedal. If the pedal still has one or two assisted applications before getting hard to press, there is likely no leak.
Another simple test is to apply the brake pedal several times without the engine running. After the pedal becomes hard to push, hold it down and start the engine. A good booster, with adequate vacuum will normally allow the pedal to drop slightly. This is because the booster has greatly increased you pedal effort.
When brake boosters fail the reason is normally outside of the booster
For instance a bad master cylinder can leak fluid into the booster. Brake fluid can deteriorate the diaphragm causing failure. Worse, the failed booster can provide vacuum to the rear of a replacement master cylinder and quickly ruin it, repeating the cycle.
It is wise to closely inspect the booster and the master cylinder when either fails. As with most things, careful diagnosis can prevent expensive re-work. If you have power brake problems, let the professionals at AGCO diagnose the problem. AGCO, it’s the place to go!
