>If you didn't have properly operating pressure maintaining operative on a 26L, you couldn't legitimately pass an initial terminal air test, cause when you made a set, and allowed it to equalize, before cutting out the valve to test train leakage, it would just continue to leak (bleed) down (bad gasket/baffle, if I recall correctly?).

Yes, as explained to me by a WABCo repairman, early production units of the 26-L brake valve were prone to this, and it had to do with one of the materials used somewhere in the high capacity relay valve apparatus wearing out or hardening (some sort of bushing material, but it's been a while . . . ). The fix was to use an improved material. I had this happen once too and had a divil of a time convincing the roundhouse forces the brake valve was defective because it would do it when the train was attached and the angle cock open, but if you closed the angle cock behind the engines, it wouldn't. Being roundhouse types, they thought in terms of "locomotive air brake test" instead of "train air brake test", and it was the latter which mattered (not to mention specified by the Power Brake Law).

Another problem the 26-L valve can have is that the high capacity relay valve's diaphragm can rupture, causing complete loss of control of the automatic air brakes, although this isn't a very common occurrence. It can also happen to the high capacity relay valve in a 30-CW module, which is the same apparatus, detached from the brake valve and placed in a different location on the locomotive.

The most famous and humorous incident of this type of failure in my area involved an Amtrak train with and F-40. The engineer was stopping for a red block signal (number plate) and the diaphragm gave way, sending him by the signal -- this was in the days when you had to stop for red block signals. Being the rather excitable type, he got on the radio, wildly exclaiming "I can't stop the train, the brakes have failed, I can't stop the train!" Meanwhile the conductor is back in the coaches frantically trying to put his hand over the speaker of his pack set to keep from alarming the passengers needlessly (the train was going all of about three or four MPH when this happened). The E finally calmed down a bit and stopped the train with the jeff.

For our readers: cutting out pressure maintaining on a 26-L is accomplished by cutting out the brake valve. This can also be done with any other system which has pressure maintaining; using the lap position is not always an option for doing so.