The above reply by wa8xym describes the loss of air cushion in your water heater and where my thought process was headed on your pump issue. When the air cushion is lost, the freshwater system is jammed full of water with no room for heat expansion when the water is heated. When the heater starts up, the pressure can build close to 150 psi and trip the safety relief valve to weep off the excess expansion pressure. If this happens, you can see calcium and mineral buildup on the relief valve discharge port from the weeping. This high pressure will drop off instantly when a faucet is opened as that action burps the system.
Even when the system is cold, if and when the air pocket is gone, even without heating the water, the pump has to hit a dead-end filled system and can trip the pressure switch on the pump.
Is your water heater a Suburban or an Atwood? The Atwood is easy to put a drain valve on; the Suburban, which uses an anode in the drain port, makes it complex to add a drain valve. Even Atwood/Domentic discusses the air cushion loss and relief valve weeping in the heater instructions of the more modern heaters. Your 2004 manual may or may not mention it. Finally, the new Dometic redesigned water heater instructions talk about adding an expansion tank to cure the issues. They did not supply the expansion tank, they tell you to add it.
This is an age-old issue in the RV industry. If they would add an expansion tank from day one, the problem would not exist until the bladder leaked in the accumulator then there is no expansion left once again
Hope this helps and let us know how this comes out. Check your relief valve on the water heater for mineral buildup also.
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.
