City water filling fresh tank

JJinVista wrote:
augustpilot, what brand / model of pump are we talking about?

I have a like-new pump, a Shurflo 4008 Revolution, and it's exhibiting the same problem with water flowing backward through the pump.

I've had it apart a couple of times now. I've identified the check valve which is located within the pump, and inspected the check valve seals, o-rings, and for signs of debris or grit. Nothing appears worn, defective or dirty.

I know water is flowing through the pump backward because I've disconnected the suction side of the pump (once it was primed), had the pressure side connected to city water and have seen it drip out the suction side at a rate of around 6 drops / sec.

What's odd is that it inconsistently does this.

If the pump isn't primed (as when I had it out of the RV, hooked to a garden hose on the pressure side), it doesn't fail / backflow.

With it hooked up in the RV and the pressure side of the pump pressurized with city water, it doesn't fail as long as it's not primed.

I ran the pump for a few seconds like this (dry), and it doesn't fail. (The manufacturer says this pump can be run dry without damage).

Only once the pump is connected to a water source on the supply / suction side, primed and a few cycles of turning a fixture on and off, causing the pump to cycle, will it then start to leak backwards from the pressurized side to the supply side.

This leaking condition persists, even when the city water supply / pressure is not present, and even after opening a fixture valve to take all pressure out of the system. Granted, the leaking condition would eventually stop when there's no more water in the plumbing lines to backflow, but it appears that simple gravity and / or siphoning action is making this happen.

I feel that maybe this particular design, brand or model pump is prone to this problem. Or else maybe there's a defect in this particular pump's housing so it doesn't matter what condition the o-rings and gaskets are in.

I'd like to hear from others as far as what brand / model pump has had this problem, the age of the pump at the time of the problem, and if a check valve rebuild kit fixed the problem.

Not sure I want to waste my time getting a check valve rebuild kit because even if it fixes the problem, it appears that this pump's check valve design can't tolerate much wear before the condition will begin again.

Maybe I'll just add an external secondary check valve, as the pump itself works great.