The booster doesn't even do anything when there's no vacuum from the engine not running. All of them have a two-piece mechanical link that never operates unless there's a loss of vacuum; then it is the safety backup. You can prove that by stopping the engine, pumping the pedal three times to exhaust the stored vacuum, then you'll see the pedal is very high and hard. Also, when you have the master cylinder off, have a helper push the brake pedal and you'll see the push rod come out of the booster. That shows it is pushing on the piston inside the master cylinder. A booster that becomes defective while it's on the car will cause a high and hard pedal.
If people are trying to convince you the booster is the problem, remove one of the steel lines, then watch how much fluid comes out of the port when someone pushes the pedal. You can also screw in plugs into both ports, then you'll find the pedal will hardly move. Be careful doing that though because you'll risk damaging the threads or the seat the lines seals against.
It IS possible to have a problem with the booster causing a low pedal but only if something is assembled incorrectly, not once it's been working.
Aside from the obvious air in the line, a low pedal can also be caused by a leak, by rear drum shoes not properly adjusted out, and by a sloppy front wheel bearing. With a sloppy wheel bearing, it has to be so bad that it was making noise for quite a while. The clue is the brake pedal will be high and firm when the car is standing still but it will go close to the floor the first time it's pressed after driving a while, often on bumpy roads. The wobbling rotor pushes the piston back into the caliper. You have to push the pedal unusually far to push that piston back out before any fluid pressure will build up.
As for bench-bleeding, you can get away without doing that but then do-it-yourselfers think they have to bleed at all four wheels. That is a whole pile of air that has to be moved out, and that's more work than any mechanic wants to go through. Ten minutes of bench-bleeding can eliminate an hour of bleeding at the wheels and lots of fluid.
Another trick if you think there's still air in a front line is to use a screwdriver to pry a piston back into the caliper as far as it will go. That will push the brake fluid up into the reservoir and wash any air in that line up with it. You can even have someone watch in the reservoir to see if any air bubbles show up. It doesn't take much air to make the pedal go all the way to the floor.
By the way, when doing any pedal bleeding with a helper, never allow them to push the pedal more than half way to the floor because of that corrosion, but there is one exception. That's with your new (rebuilt) master cylinder. It takes a year or two for that corrosion to develop. Before that occurs it is okay to push the pedal all the way to the floor. That is similar to what you're doing when you bench-bleed it.
The only way to get a new master cylinder is through the dealer, and that is normally only done when the car is under warranty and the manufacturer wants their parts to be used since they're paying for the repairs. You will only find rebuilt units at the auto parts stores. You can also often find rebuild kits but they are not a good choice for multiple reasons. First of all, a professionally rebuilt master cylinder with a warranty is almost always less expensive than the rebuild kit. To do it yourself, it takes a lot of time, costs more, and if you mess it up you pay and do it again. Third, since the late '70s all master cylinders are made of aluminum to save weight. Since aluminum corrodes and forms aluminum oxide within seconds of being exposed to air, the bores are electrically coated with an anodized coating. We used a hone on an electric drill to clean the bores in older cast iron master cylinders, (and wheel cylinders), but with aluminum ones you can't use anything that will scratch that coating. Only brake cleaning chemicals and rags are acceptable.
Sunday, September 2nd, 2012 AT 6:29 AM