Water coming OUT the IN

mockturtle wrote:
Or, you can get a screw-on cap to put over it when you're not using it.

That is what I carry in my spare parts/junk bag. Never had to use it since the check valve failure that resulted in it's purchase. :h

Just add a garden hose shut off and forget about it.....those check valves are junk. Plastic plunger/spring/o-ring junk.

Or, you can get a screw-on cap to put over it when you're not using it.

Your city water inlet has a check valve in it that when working right allows water to enter the system when hooked up with a hose to a city water source. And then it works to close off the flow of water from coming out of the inlet when you use the water pump. The inlet needs to be replaced. Here is a link of what it looks like
Water inlet check valve

Undo the three screws, and inside there will be the check valve. It's inline with the fitting and only just under one inch long. Just unscrew it and replace it with a new one. They are very inexpensive, only a few dollars.

The check valve is supposed to seal, and allows pressurized water to go in, but then will seals and will not let any out.

Usually the O ring inside gets out of it's track. They are a pain to fiddle with, and easy to change.

Temporarily, you can put a plug in the line, or a tap on a short hose until you fix it.

Bad check valve on the city water inlet line.

Rusty