Air-Assisted Power Brake Booster:
Many vehicles utilize a power brake booster to assist the driver in applying hydraulic pressure to the master cylinder and the brake calipers or wheel cylinders. These boosters operate using the vehicle's engine vacuum.
Vacuum Operation:
When you press the brake pedal, you engage a series of mechanical systems culminating in a piston movement towards the vacuum. In this scenario, a brake-activated check valve opens, and it creates atmospheric pressure on the booster diaphragm's backside. Simultaneously, this allows the engine vacuum to act on the booster's front side. Thus, vacuum assistance enhances the driver's pedal effort through mechanical advantage.
Air Leak:
If you hear a hissing noise when the brake pedal is released that stops when you press it, there is possibly an air leak within the booster diaphragm (the large rubber-like barrier inside the booster assembly) or a faulty check valve. As vacuum pressure builds up while depressing the pedal, it temporarily seals leaks within the booster and hence, the noise disappears. Upon pedal release, air may reenter through holes or punctures on the diaphragm.
It's crucial to check and address this concern since an air leak diminishes power brake assistance efficiency and may impact overall braking performance and driver safety. This sound typically signifies the imminent failure of the brake booster mechanism and demands proper diagnostics and possibly booster replacement by a qualified auto mechanic.
