[Fulltiming]

They are located on the air brake tanks themselves. They are check valves to prevent air from leaking out of the tank back into the supply system. Although it is possible that they are leaking where it is visible, they can have debris or rust preventing them from properly sealing and allowing the tanks to bleed air back into the supply system. If they have not been changed out in a significant period of time, it is a good idea to change them IF you watch your gauges and see one or both brake tank pressure gauges dropping at about the same rate as the supply tank.

Bendix 800372 SC-3 on my coach.