Do semi's have a switch to release air pressure?

Question

Almost everytime I pass a semi on the highway or interstate, just as i'm reaching the cab I hear a loud burst of air coming from the semi i'm passing. Are some of the drivers doing this on purpose, by hitting a switch in the cab.

Answer 1

They have a pressure relief valve that releases a blast of air automatically when air pressure reaches a certain point. This is so the truck doesn't build too much pressure. Sometimes it is called a "sneezer".

Answer 2

Most big-rig trucks use air brakes that release compressed air when the driver steps on the brake pedal.  Some air compressor systems also have automatic vent systems that will blow off a bit of air to drain moisture from the bottoms of the vehicle's air storage tanks.

Answer 3

No, semis need constant air pressure for brakes etc. The compressor has a safety device to release excess pressure which  builds up. The driver can set the brakes on the trailer and this causes a loud burst of air, but thats done usually when parking the rig.