Brake booster problems can be really dangerous. A few common problems that are difficult to detect, but commonly occur include the following. Hold the brake pedal down and start the engine. The brake pedal will pull down slightly when the engine starts. If this does not happen, it indicates a problem with the car vacuum. Vacuum or fluid leaks from the booster are possible due to worn out or damaged pistons, valves and hoses. There is a large internal diaphragm that creates the vacuum for the brake. It can harden with time, and this can cause your brake to harden and be difficult to press down. As a precaution, check the manufacturer’s manual for instructions on the type of brake boosters you are using.
For more info, see 4 Steps to Repairing a Brake Booster