[Richard]

I think, and think is the key word, that this is the valve that instantaneously releases pressure to the brakes when you take your foot off the pedal. Have a partner confirm by applying and releasing the brakes a few times. You should get a rush of air out of the valve if its the thing I think it is.

You might also check the brake pedal valve. If it is leaking through it's body, it could deplete all the tanks.