Yes, air parking brakes are spring set and air release. The driving brakes are air set, spring release. The sound heard at the ramp needs clarification....was it the usual "whoosh", followed by silence, or a continuous sound of air escaping? It might've been an air dryer purging the system, as they do regularly.
If you stop the truck by depressing the brake pedal, and hold the pedal down, the truck shouldn't move. If an air line blows or otherwise compromises the system, you will hear a continuous leak as the pressure decreases. When it falls low enough, the truck will begin to roll if it's on a grade. When it falls below the pressure exerted by the parking brake spring, the parking brake will begin to set. If it's in good condition, it will reach the point where the brake is locked and holding the truck, provided the leak continues to cause the air pressure to decrease.
If the driver got out and didn't set the parking brake, then the truck would be free to roll.The sound described by the witness should have no bearing on making the truck roll, as the parking brake is not set anyway. Something doesn't add up.
