It’s not the power brake booster.
** Power brakes are a universal feature on current vehicles, and nearly all of them rely on engine vacuum to produce their power assist. The vacuum brake booster between the brake pedal and the master cylinder is the last possible source of a low brake pedal that we’ll discuss here.
On a brake system that does not have a power assist, the brake pedal is directly connected to the master cylinder. When a manual brake system is working properly, the brake pedal always feels high and hard when it’s applied. This changes in a subtle way when there’s a brake booster between the brake pedal and the master cylinder. The pedal should still feel solid while braking, but it’s normal for there to be a small amount of pedal travel before the brakes begin to apply. This is to be expected—it’s just the brake booster doing its thing.
A quick check of the brake booster must be conducted with the engine off. Pump the brake pedal slowly several times. This will gradually deplete the vacuum assist stored in the brake booster. Once all of the vacuum has been depleted, the brake pedal should feel as high and hard as it’s going to get. However, the vehicle would be a good deal harder for the driver to slow and stop in this condition. If the pedal doesn’t feel high and hard, you’ve eliminated the brake booster as a potential source of the problem and will need to look elsewhere.
If the brake booster has failed, it would cause a symptom that’s just the opposite of what we’ve been discussing here. Rather than complaining of a brake pedal that’s too low, the vehicle owner would be complaining of a brake pedal that’s high and hard, and a vehicle that requires a good deal more pressure on the brake pedal to bring to a stop because there’s no power assist.— Karl Seyfert**