Originally Posted by AGreen

No, the brake booster only assists your foot in pushing the pedal. The car will stop without it, it'll just take more leg force. Brakes work on a hydraulic principle. In a nutshell, you push the pedal, the booster assists in pushing the master cylinder. The master cylinder therefore puts a pressure on the system. Fluid pressure through the lines puts a pressure on the calipers to squeeze the brake pads to the rotors. The calipers are essentially pistons that get pushed by the pressure from the master cylinder.

The whole system is sealed. Air does not get in, fluid doesn't get out. Air is compressible, and for this discussion's sake, fluid is not. So if air gets in somehow, it will be compressed, preventing the pistons from moving.

Now, here's where it gets more complex. Brake systems since the 60's have been split. The master cylinder has 2 pistons inside. This is because car manufacturers realized it's too risky to rely on 1. If one line gets a hole in it, you won't lose all braking force. Depending on the car, it splits the system diagonally (one front wheel, one rear wheel) or front-to-back, meaning you'll still stop on either the front or rear brakes. So even if your master cylinder "went bad," it's unlikely that you've lost all braking force. Unless you blew out both seals at the same time, it's highly unlikely. So what has probably happened is a leak developed somewhere, and the MC reservior went dry. Air was introduced into the system, and the brakes no longer work.

Even if you just add fluid, it won't work again because you have to "bleed" the brakes. That means you have to get all the air out of the lines. This is best left to doing a google search, because I could spent a loooong time explaining all the little details. Start by bleeding to get an idea of where you're at. Once that's done, we'll see what happens from there.