Most cars and trucks have vacuum-assist power brakes. If you've noticed the brake pedal seems harder and requires more effort to apply the brakes, you may have a problem with the brake booster.
To check the booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit (the pedal will feel firmer and you won't hear any sounds from the booster). Then hold the pedal down and start the engine. You should feel the pedal depress slightly as engine vacuum enters the booster and pulls on the diaphragm. If there's no change, the vacuum hose to the booster may be loose or blocked. If the vacuum hose is okay, the problem is in the booster and the booster needs to be replaced.
If your brake booster has failed, your brakes will still work but will require increased pedal effort. The pedal will feel much harder and will take a lot more pressure to stop the vehicle. Driving with a bad booster can be dangerous because the vehicle may not be able to stop as quickly or in as short a distance. So don't delay. Have the problem diagnosed and repaired as soon as possible.
If your vehicle has an "integral" antilock brake system (where the ABS system is combined with the master cylinder assembly), power brake assist is provided by pump pressure stored in the ABS accumulator. (NOTE: If your vehicle has a "nonintegral" ABS system, it has a conventional vacuum brake booster.) If there's an ABS pump or accumulator failure, power assist will be lost and the ABS warning light should come on alerting you that a problem has occurred.
WARNING: If the ABS warning light is on, the ABS system is usually deactivated which means the ABS system can't prevent skidding when braking on wet or slick surfaces. You should have the system checked out and repaired as soon as possible.
Though not as common as vacuum booster power brake systems, some vehicles are equipped with Bendix "Hydro-Boost" power brakes. This system uses hydraulic pressure generated by the power steering pump rather than engine vacuum to provide power assist.
Pressure generated by the power steering pump is stored in an accumulator, which is then routed to the master cylinder by the Hydro-Boost unit when you step on the brakes. Problems can be caused by leaks inside the Hydro-Boost unit, by a worn power steering pump, slipping or broken pump drive belt, or hose connections.
A simple way to test the Hydro-Boost system is to pump the brakes five or six times with the engine off to discharge the accumulator. Then press down hard on the pedal (about 40 lbs. of force) and start the engine. Like a vacuum booster, you should feel the pedal fall slightly when the engine starts, then rise.
The leakdown of the accumulator can be checked by pumping the brakes several times while the engine is running, then shutting it off. Let the car sit for about an hour, then try the brakes without starting the engine. You should get 2 or 3 soft brake applications before it takes more effort to push the pedal.