Inside the Triple Valve - Part III

With the aid of a large socket wrench, we have here a disassembled quick action portion of the M valve. There's a spring loaded shaft pushing on the small cylinder, so that it does not activate unless the brake pipe pressure drops rapidly.

The book says not to lubricate the quick action parts, since they are used so seldom. But they should be cleaned, of course. It's important to check that the parts seat correctly; if they don't, it can open a direct leak from the brake pipe to atmosphere.

And it's a good idea to check the emergency feature before starting service each day. You should be able to hear the quick action clearly, as the brakes set rapidly and each valve in the train dumps air to the exhaust.