Automatic Air Brakes for riding scale rolling stock

The system relies on the fact that the check valve to the car brake reservoir has a 'cracking pressure' (i.e.: pressure loss) of about 2 psi and the 'rod end' of the cylinder has a smaller effective area than the piston end.  Due to the check valve loss the car brake reservoir pressure will be approximately 2 psi lower than the 'full release' train line pressure.

The lower car brake reservoir pressure and the smaller effective area of the cylinder rod end allow the piston to be forced into release (as opposed to just going slack) when the train line is at the 'full release' setting (40 psi in my case) so no 'hold off' springs are required to keep the brake shoes from dragging.

In the 'Full Feature' system the 'Normal/Bypass' switch allows the car to be cut out of the train line if there is a malfunction on the car (leak, etc.).  The 'Normal/Release' switch is used to force the brakes off in the absence of train line pressure (e.g.: the car is disconnected from the train) allowing the car to be moved.  In practice, the car brake reservoirs on my cars (2" PVC pipe about 30" long) allows the brakes to be released and applied many times before it is exhausted.