Hey Dave and Dave! I'd like to solve the puzzle! (Maybe
Water in the trailer's plumbing can be pressurized by either 1) the pump or 2) connecting to what is commonly referred to as the "city water" connection. Mind you, this "city-water" connection can actually be connected to non-municipal water systems, such as the Rest-Your-Bunzawhile trailer park's water system.
Pressurized water in the trailer's plumbing will press against and close not one but two check valves. (Check valves are valves that only allow flow in one direction.) One of these valves is integrated into the water pump's diaphragm, and the other is located at the "city water" inlet. The check valve in the pump prevents pressurized water from draining back into the on-board gravity water tank. The other prevents pressurized water from entering the "city water" system.
Pressurized water in the trailer plumbing can contribute to making these valves stick, at least the one in the pump; I don't actually know how the check valve in the "city-water" inlet is configured. In my case, while I have never connected my trailer to the city water connection, I have been parking the trailer for extended periods with the plumbing pressurized; I have never made a point of opening a faucet and draining off the residual pressure when I return from a trip. So, the pressurized water presses against the check valves for months at a time as I have a very tight system with no leaks
. (Don't try this where you have "actual winter" folks!)
I think what we are getting out of this is that the pump check valves tend to stick closed and need a little "encouragement" to open. Jerry also mentioned this in post #9:
Several folks have mentioned letting the pump run a bit. That's clearly got some promise as it has worked several times. However, I think Dave Fish's idea of creating a source of suction is most excellent. This would "pull" open the sticky check valve in the pump.
The other thing I get out of this is that I should be opening a faucet and draining off the residual pressure when I park the trailer. This will prevent pressurized water from pressing on the check valves for long periods. The check valve seats in the pump are "rubbery", so I can see that having these valves pressurized for long periods will encourage them to stick.
Thanks to all for posting their experiences. I sort of kick myself here as I work with and specify pumps for different applications in municipal water systems. On the other hand, I have to allow that these trailers, their equipment and the operating conditions are all new to me. Bottom line, it's been great for me to learn from others here on the forum.