Jim M
Because it leaks out while thw truck is parked,

Mike S
Air brakes are meant to work that way. The air pressure is what holds that break open without air pressure the brakes lock. That is so when you are driving a rolling house down the road and you have a brake system failure your truck is still going to stop eventually..

locksmith
The air pressure unlocks the trailer brakes and allows the wheels to turn. When there is no air pressure, the wheels are locked.

sidecar0
Old and Weak Air Lines, Bad Check Valve at Air Tank Inlet from Compressor, Small Leak at Foot Valve, Bad Spitter Valve (Air Drier), Leak at Tank Drain Valve.