It helps build up air pressure quickly without running the engine. It can be called the foot valve or the relay valve. It can be called the foot valve or treadle valve. It can be called the relay valve or air compressor valve. The brakes will be cooled down as soon as all the hot air is removed.
Air pressure can build up until the brake pressure gauge is broken.
Air can be let out of the system faster than the compressor can replace. At each wheel. On every other axle. On the drive wheels only. Directly beneath the foot valve.