Brake valve

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Air Brake Valve

Big Dave posted on Chaski.org

A typical air brake valve has 5 positions:
* Release - puts the engine compressor to charge the train line to operating pressure.
* Lap - cuts it off, and is used with the service position to hold a brake reduction.
* Service - is just that, and has the caveat of bunching the rear of the trains and making a slack run in/out, as air moves take 13 cars a second or so to make the pressure changes.
* Rapid service
* Emergency (The Big Hole) - is a dump that causes the air valves to sense a big change and dump the train line at the car, for the most rapid engagement of the brakes. Since all cars are not equal, this can cause its own problems, as each car may not apply as well as the next. Once Emergency brakes are set, the train line must be dumped to a low level, and then released to full pressure before the cars will unset the brakes. On long trains this can take a while since the air tanks on engines are not large enough to supply that much stored air.
6ET Brake Valve Pipe Diagram, drawn by John Kelso, 16 March 2016