There is no check valve per se. The switch acts as the check valve, releasing it closes off the system. There is an overfill valve that will bleed off excess pressure once the bladders are fully inflated. In order to service or find the problem it is best to remove the seat and turn it over. Check for leaks in the lines by inflating and listening for leaks. If you find a leak in the line it can be fixed by cutting at the leak and slipping some very tight fitting vacuum hose to join the 2 parts together. It maybe that the switch is not sealing but that is rare. It may also be that the bladders are leaking. The real weak point are the lines, they can get chaffed, develop a hole and begin to leak. I found a hole in each line on my 89. I found both of my leaks where the line goes into the side bolsters and the lumbar cushion. It comes from underneath the seat up by frame to the seat back cushion. Just follow the line. It is partially hidden by a piece of carpet at the back of the seat bottom. That carpet just snaps on. Pull it off and it will expose more of the line.