Let us imagine that a train has been standing in a siding, and that air has gradually filled the vacuum chamber by leakage. The engine is coupled on, and the driver at once turns on the steam ejector,21 which sucks all the air out of the pipes and chambers throughout the train. The air is sucked directly from the under side of the piston through pipe D; and from the space A A and the cylinder (open at the top) through the channel C, lifting the ball, which, as soon as exhaustion is complete, or when the pressure on both sides of the piston is equal, falls back on its seat. On air being admitted to the train pipe, it rushes through D and into the space B (Fig. 86) below the piston, but is unable to pass the ball, so that a strong upward pressure is exerted on the piston, and the brakes go on. To throw them off, the space below the piston must be exhausted. This is to be noted: If there is a leak, as in the case of the train parting, the brakes go on at once, since the vacuum below the piston is automatically broken.