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.
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 pushrod 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.
thanks jim,,good read. what would other causes of low vacuum, below 16", be if the motor was good and no other vacuum leaks? would a large cam or a single plane intake cause low vacuum reading and poor booster performance? thanks,,john
I have not heard of or experienced low vacuum problems caused by large cams or single plane intakes. Hypothetically, it could happen. Realistically, it's one of the other causes.
A large cam will decrease vacuum noticably, with 220-230 degrees at .050" having around 12" vacuum at an idle. That amount of vacuum should be enough to run the power brakes.
I have used cams with 230/240 duration without loosing the power assist, even though Pontiacs with their 30 degree intake seats typically behave as though the cam were another 10 degrees "bigger" in other engines.
Most "streetable" cams will result in at least 8" of vacuum. Below that and you start loosing the signal needed to pull fuel through the carb accurately at an idle and at low speeds.
The intake has more to do with what range the engine likes rather than how much vacuum it makes.
Quote, "Hypothetically, it could happen" That's my disclaimer. Quote,"would a large cam or a single plane intake cause low vacuum reading and poor booster performance?" Seems hypothetical or retorical or an enigma. A question wrapped in an answer.... Should have said simply, "Yes". You can lead a horse to water but you can't make him drink.
thanks guys,,you answered my question very professionally. i'm still a little confused with TOHCan's statement, i understand the brake booster needs 16" to operate properly, TOHCan stated that 12" at idle, should be enough to run the power brakes! would the brake pedal have a "hard" feel at 12"?
I don't think it would. I think the power assist would gradually fade, not drop right out at 12. Unless it's some special aftermarket booster that works different than the originals. That would be a safety problem if you think about it. As soon as you get a slight vacuum leak your brakes start working funny. No, there would seem to be a problem with the brakes.
You're very welcome, John. Jim's right, as the vacuum gets lower, the power boost is reduced as well. It's the pressure difference between what your engine makes for vacuum and the local atmospheric pressure that determines how much assist you get.
However, I installed a used booster on one of my other cars, and ended up with way too much boost! The brakes were extremely touchy and almost impossible to use properly. I installed yet another booster and the brakes were back to normal. I haven't run across that problem since. I doubt that the booster was built differently regardless of the origin, so I think the valve that opens when you press on the pedal was probably defective and opening too easily.
Quote, "Hypothetically, it could happen" That's my disclaimer. 
