I have a macerator pump that is two years old. It works fine but when the head in not being used and the valves are closed there seems to be some water entering the bowl. The intake valve seems to be okay because when it is closed the water level will only rise a couple of inches in the bowl. If that intake valve is open the bowl will fill completely. Is there some type of check valve on the exit side of the system? Also, the water that enters the bowl does not appear (or smell) like clean water. I've tried this with the holding tank full and empty. When the tank is empty it seems to fill more slowly than when it is full. I don't see any obvious check valve but can't imagine there not being some system or valve to keep sewage from back-washing into the bowl. Oh yeah, it is a Jabsco macerator, not sure of the model number. Any input would be appreciated, thanx.