Pneumatic systems on a truck/bus are brakes, both service and parking ones, power shifting of the gearbox ( very common on ZF, MB and MAN gearboxes amongst others), power clutch for manual gearbox ( you just press to disengage, and release the pedal to give a controlled smooth application as the system bleeds air out at a controlled rate), wipers, suspension ( both air bag suspension for trucks and ride height control for buses), door openers for buses and then assorted ancillaries, like a wheelchair lift on some buses, retracting steps on others and on some the windscreen washer as well.
This is all supplied by an engine driven compressor, filling a few pneumatic reservoirs, and also include a manual or automatic drain valve to release condensed water from the air, inlet and outlet filters for the compressor, oil coalescing filters to get the dirt and oil vapour from the compressor out ( it is fed in many cases from engine oil for lubricating the pistons, bearings or other parts like variable displacement devices, and can have oil traces in the outlet air), some pressure burst disk, a emergency pressure valve ( burst disk is if the emergency valve fails to operate) and a pressure regulating valve that dumps excess air pressure when the compressor has filled the tanks to operating pressure. Thus the regular dumping when idling.
Brake systems generally require pressure to release ( aside from some older systems grandfathered in decades ago and generally now required to be modified) so that if you do not have enough pressure ( big red brake warning light in the console) and attempt to release the parking brakes ( pressurise the system over 2bar with main system still under 6bar) there is a loud warning buzzer in the dashboard to tell you brakes are still applied. All other sytems require pressure to operate, just the brakes must be fail safe with burst piping or disconnected trailer lines.
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