[Richard]

Adrian, the most common failure is that the spool that shuttles back and forth in the valve starts to leak by. This is the type of shuttle valve that keeps pressure at all times on the cylinder. So when in closed position air pressure is applied to the dump valve to keep it closed, and the opposite is true.

I am commenting for posterity, so that when one is chasing air leaks in this setup, be aware that the spool in the valve body is the most common leak, BUT the orings in the cylinder can also leak.