A sump pump operates by pumping out ground water and rain water from basements and sub-basements to prevent flooding. Many sump pumps have check valves. On some models the check valve prevents the backwash of pumped out water back into the sump pit. On other models where the discharge pipe connects to the sewer system, the valve prevents sewage from backwashing into the sump pit. Over time the valve can develop leaks either due to loose connections to the pipes or a failing internal valve.
Locate the check valve housing. It's made of metal or hard plastic with multiple bolts holding it in place. On some sump pumps the check valve is on the top of the pump with two pipes running into the valve. On other pumps the valve is attached to the discharge (outlet) pipe running towards the sewer line or outer wall.
Inspect the center connection of the check valve for a leak. Inspect the spots where the pipes (especially the discharge pipe) connect to the valve. Feel around the valve for moisture or dripping water.
Slowly pour 5 gallons of water down into the sump pump pit. The pump will kick on and begin drawing water out. Watch the check valve connections to the pipes and the valve itself as the water drains.
Make note of exactly where the leaks are occurring on the check valve. Wait until the pump shuts off. Look in the sump pit to see if water leaks from the intake pipe. If it does, this can be a sign that the check valve's internal valve is failing.