I have a washing machine and a slop sink that basically connect to the same sewer line. The washing machine pumps directly into the sewer line and it does have a check valve. From the exit of the machine it has about a 4 foot riser before it reaches the check valve. For the slop sink at this time the check valve is at bottom of sink level and has about a 5-6 riser before it reaches the 2" sewer line. Almost every day some water backs up into the sink. the check valve WAS installed correctly but I think i MIGHT be in the wrong location on the discharge line. What would be the BEST location for the check valve? Thank you!!!!

If the check valve is where you state it is and the sink drains by gravity, AND the sink does drain, you have discovered a new rule of science, because the old one said "water cannot drain uphill".

The sink has a discharge pump that is manually operated. The pump has to discharge the water about 5-6 feet upward to the sewer line. The Check Valve is at the level of the bottom of the sink.

If it is between the pump and the sewer, it makes no difference where it is.

check valves designed for vertical piping are NOT the same as checks for horizontal piping

and vice versa

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

Right now the present check valve sits horizontally and water leaks back into the sink. Its a PVC check valve. I wanted to use a Brass Check valve and mount it Vertically and nearer the 2 " sewer line. Would that be better?