The brake booster in your car is one of those parts that does its job silently and unnoticed, until the day it doesn't work. Suddenly, you find that pressing on the brake pedal to stop the car takes a lot more effort. The pedal feels hard, but the brakes work very slowly, and it takes much more distance to stop.
Operation
Automotive brake boosters are operated from engine vacuum. A large flexible diaphragm within the booster has the air removed from one side when you step on the brake pedal. This causes atmospheric pressure to push on the opposite side of the diaphragm, and because of its connection to the brake master cylinder, the diaphragm helps to apply the brakes.
Possible problems
The possible problems with a brake booster are loss of supplied vacuum or a cracked diaphragm.
Troubleshooting
The first step to troubleshooting a problem with a brake booster is to verify that a problem exists. With the engine off, pump the brake several times and hold the pedal down while starting the engine. A working power brake will pull the pedal down a bit further; if the pedal does not move, the next step is to check that engine vacuum is drawing on the booster. Check the hose for cracks and breaks and also that it is connected tightly at both ends. With the engine off, remove the hose end from the brake booster and the plastic valve from the hose end. Blow into the valve; it should allow air to flow in one direction only. Replace it if you can blow into both sides.
Start the engine and check for vacuum in the hose. The engine may run rough when the hose is unrestricted but will smooth out if you plug the hose end. If no vacuum is felt, the problem is a restricted vacuum port that must be cleaned before further testing.
Attach the valve and hose to the brake booster. Listen for air leakage, which could indicate a cracked booster. Have a helper apply the brakes while you listen also.
If adequate vacuum is being supplied, no air leaks are heard and the brake still has no power assist, replace the booster.
