This is a Westinghouse M2A triple valve with graduated release, quick recharge, and quick action features. We'll see what that means later.
Here the piston assembly has been pulled part way out, so you can clearly see the piston itself. It has a brass slip ring to seal it to the cylinder. Diameter is 3 1/2".
And here's a view of the cylinder. You can see the square cutout at the bottom; this is where the slide valve moves, and you can see some ports in the floor of the cylinder which line up with the slide valve as it moves back and forth.
3. For a moderate brake reduction, such as 10 psi, the pressure in the auxiliary at some point will drop slightly below the brake pipe. The piston then moves in, taking the graduating valve with it, but stopping when it hits the slide valve. We are now in Service Lap and all ports are closed.
To increase the braking force, a further reduction in brake pipe pressure can be made, so the triple goes back to Service, then Service Lap again.
The parts are proportioned so that when the auxiliary drops to about 50 psi, the brake cylinder has risen to 50 psi, and they are equalized. This is the maximum brake pressure you can get in Service position. Any further (service) reduction will accomplish nothing.
4. To release the brakes, the brake pipe pressure is increased. The triple moves into Release and Recharge, and the auxiliary pressure starts to increase. If we don't want to release all the way, but just reduce the brake cylinder pressure, that's called graduated release. The auxiliary pressure will rise above the brake pipe because the control pipe always supplies 70 psi, and the piston will move out slightly. But this time, it stops when it hits the slide valve, and we are now in Release Lap. The port connecting the brake cylinder to exhaust is closed, so the brake cylinder pressure stops decreasing. Without the control pipe, graduated release would not be possible. We can partially reduce the braking pressure by increasing the brake pipe, and the triple will move between Release and Release Lap.
5. Finally, there's the quick action feature. In an emergency, the brake pipe pressure is reduced rapidly. and this causes the spring-loaded quick action portion to open. This puts the valve in the Quick Service position, otherwise known as Emergency or Dynamite or "Oh NO!". The quick action valve dumps air from the auxiliary reservoir and the control pipe into the brake cylinder, so that we get a maximum brake pressure of 70 psi. It also dumps air from the brake pipe to atmosphere, thus helping to quickly reduce brake pipe pressure along the train.
Well, I hope that was educational. Be sure to study this carefully - there may be a pop quiz soon!