I am a long time virtual trucker and one thing that happen when you try to wiggle your way around the yard is you use up all the air in the tanks and the breaks lock up. what fixes this is having to sit there for a bit idling to build it back up. Thats why then trucks use also an engine break where basically the engine is forced to rev down and slow the truck down with it. That would be part 2 of this idea. With the simple air break simulation the truck will now push out excess air through a saftey valve. this happens automatically when the tanks have reached the capacity. Depending on the condition of the valve it will depend on how often you will hear that air ejection, lenght,etc. Depending the truck, you will hear the air compressor working hard or rather quiet. Older trucks tend to have a noisy air compressor. Air used to apply the parking break as well, so there's a few places its need it. I mean if you abuse the T-series she will get cranky.
