system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 Why do trucks and busses do the “tsshhh” thing when they stop?
gpt
As others have said, large vehicles such as busses use compressed air to apply pressure to the braking pads that stop the vehicle. However this doesn't quite explain the hiss on stopping because if the brakes are on while stopped then surely they want to *keep* air pressure, not release it right?

The reason this happens is because there are actually two braking systems at work, the usual driving system that uses air to apply braking pressure and the parking brakes which apply pressure with springs. While driving normally the parking brakes are held open with air pressure, but if for some reason pressure was to drop below a critical point the parking brakes could close under spring pressure.

It is an automatic safety feature just in case the vehicle brakes fail, but this also is how the parking brakes are normally applied. Normal procedure is to apply the parking brakes at every pickup, and to do this the pressure in the lines between the storage tank and brakes must be released. The hiss you hear is that happening as the parking brakes are applied.