Brake pedal feel is firm, but brakes lack sufficient stopping power or fade
Check the operation of the brake booster and brake booster check valve. Replace worn or failed parts.
Check brake linings and brake surface areas for glazing and replace worn or damaged parts.
Check for seized hydraulic parts and linkages, and clean or replace as needed.
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.
How can something take a lot of nothing? After all, vacuum is the absence of atmospheric pressure. The answer is the higher the vacuum, the greater the atmospheric push to fill the void. As some famous scientist once said, "Nature abhors a vacuum." What he meant was as soon as you create a hole in the air (vacuum), the surrounding air tries to rush right in to fill it back up. So the push that a vacuum brake booster gives to the master cylinder is actually atmospheric air pressure.
At sea level, air pressure is 14.7 lbs. per square inch, which equals about 30 inches of vacuum on a gauge. The average engine can't pull that much vacuum, so the most intake vacuum you're going to read is about 20 to 22 inches. Most engines pull a steady 16 to 20 inches of vacuum at idle. One exception is a diesel, which has no throttle to create a restriction and thus no intake vacuum. So diesels have to use an auxiliary vacuum pump if they have a vacuum brake booster.
How the brake booster uses vacuum to provide power assist is amazingly simple. The original "Master-Vac" power brake booster that became the predecessor to virtually all vacuum boosters today was patented back in the 1950s by Bendix. The booster housing is divided into two chambers by a flexible diaphragm. A vacuum hose from the intake manifold on the engine pulls air from both sides of the diaphragm when the engine is running. When the driver steps on the brake pedal, the input rod assembly in the booster moves forward. This blocks off the vacuum port to the backside of the diaphragm and opens an atmospheric port that allows air to enter the back chamber. Suddenly, the diaphragm has vacuum pulling against one side and air pressure pushing on the other. The result is a forward push that helps shove the push rod into the master cylinder for power assist.
The amount of power assist that's actually provided by the booster depends on two things: the size of the diaphragm and the amount of intake manifold vacuum produced by the engine. The larger the diaphragm, the greater the boost. An 8-inch booster with 20 inches of engine vacuum will provide about 240 lbs. of brake assist.
From this, it should be obvious that the vacuum booster needs two things to do its job: a good vacuum supply from the engine, and a good diaphragm. A vacuum supply hose that's loose, leaky, collapsed or restricted may not allow the booster to receive enough vacuum to provide the usual amount of power assist. Consequently, the driver will have to push on the brake pedal harder to get the same braking as before.
A restricted vacuum hose will cause boost to drop off when the brakes are applied in rapid succession. This happens because the blockage slows the return of vacuum in the booster.
To check engine vacuum, connect a vacuum gauge to the supply hose that runs from the intake manifold to the booster. A low reading (below 16 inches) may indicate a hose leak or obstruction, a blockage in the exhaust system (plugged catalytic converter, crushed pipe, bad muffler, etc.), or a problem in the engine itself (manifold vacuum leak, bad valve, head gasket, etc.).
The condition of the diaphragm inside the booster is also important. If cracked, ruptured or leaking, it won't hold vacuum and can't provide much power assist. Leaks in the master cylinder can allow brake fluid to be siphoned into the booster, accelerating the demise of the diaphragm. So if there's brake fluid inside the vacuum hose, it's a good indication the master cylinder is leaking and needs to be rebuilt or replaced. Wetness around the back of the master cylinder would be another clue to this kind of problem.
To check the vacuum booster, pump the brake pedal with the engine off until you've bled off all the vacuum from the unit. 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. No change? Then check the vacuum hose connection and engine vacuum. If okay, the problem is in the booster and the booster needs to be replaced.
Vacuum boosters also have an external one-way check valve at the hose inlet that closes when the engine is either shut off or stalls. This traps vacuum inside the booster so it can still provide one or two power assisted stops until the engine is restarted. The valve also helps maintain vacuum when intake vacuum is low (when the engine is under load or is running at wide open throttle). You can check the valve by removing it and trying to blow through it from both sides. It should pass air from the rear but not from the front.
Replacing a vacuum booster is a fairly straight forward job. All you have to do is disconnect it from the brake pedal on the inside and unbolt the master cylinder. The push rod 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, but some late model GM and Bendix applications have zero play.
Source: aa1car.com
