I just replaced a 9 year old 1/3HP sump pump (float) this past weekend with a new pump (diaphragm switch) of the same size. There is also a backup sump pump in the sump. The discharge lines on each pump rise up 4-5 feet and then join up (kinda like a small “h”). Each discharge line has a check valve installed, about 3-4 feet above the pumps (I replaced both the original check valves with new ones). When installing the new pump, I used new 1.5” PVC piping from the pump to the check valve. I drilled a 1/8” hole about 6-inches above the new sump pump and well below the check valve.

My problem is the pump kicks in, works fine, and then shuts off. But about 30 seconds later I hear a sucking sound and then all the water in the main discharge column just falls back into the sump. I did have problems with the 1st check valves I installed (they leaked). The new ones don’t seem to be leaking. Could it be bad check valves again or what? Any ideas or help would be much appreciated. :confused: Thanks…..

Of course it's a faulty check valve that's letting water flow back inti the pit. Replace the check valve. Regards, Tom