eli5 Why do trucks and busses do the “tsshhh” thing when they stop?

Question

eli5 Why do trucks and busses do the “tsshhh” thing when they stop?

In: Engineering

Answer 1

Larger commercial vehicles are operated by an air compressor, the name speaks for itself. Once the air pressure builds up to a sufficient level, like 120 PSI for 18-wheelers, the governor will regulate when to “pop-off” signaling that the air tanks have enough supply to operate the brakes. If an air hose is leaking then a buzzer (sensor) will sound and typically you can see on the A or B gauge that you have dropped below 60 PSI which is not safe to continue driving.