Cylinder Leaks/en

Brake cylinders aren’t perfect and leak air over time. Thus, they cannot be relied on to keep the brake shoes applied for too long. This is why on parked vehicles a handbrake must be applied.

That said, if a train with applied train brakes contains a motorized vehicle with a self-lapping 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.

See also: Air Brake System Overview, Lapping, Manual Cylinder Release, Monitoring, Train Brake, Independent Brake, Brake Shoes, Handbrake