Could well be the check valve above, but also check the external shower (hot and Cold water) connections in the convience center. Make sure they are turned off as well. Found if they were on, while I drove, when I stopped and turned on the water pump, it ran to pressurize the system. This happened 3 times on one trip, till I found that I had left the cold water open. I could not find a leak but the hose and sprayer must have been been leaking.
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