Cylinder Leaks

Over time, air brake system components leak air. This includes the brake cylinders , thus they cannot be relied upon to keep the brake shoes applied indefinitely. This is why on parked vehicles a handbrake must be applied manually.

That said, if a train with a train brake applied contains a motorized vehicle with a self-lapping control valve and a functioning compressor , as long as the engine is running the pressure in all of the train’s brake cylinders will be maintained. This means runaways due to leaks won’t be possible. If the train, however, only contains a non-self-lapping vehicle doing the braking, pressure in the cylinders will not be maintained and can lead to a runaway, unless the brake handle is in the service position.