Sump pump ball check/backflow valve.

All right guys, just moved into a new house and I come to you in desperation. The home I live in is serviced by public sewer. My house is well below the grade of the road and the city sewer line(mountain country). So I have an exterior sewer sump pump to pump the sewage to the city line at road level. The problem I'm having seems to be the backflow prevent ball valve. This valve is located right above the lid to the sump tank where the discharge pipe comes from the sump pump. It is a pvc ball type check valve with threaded clean-out lid. Three times now the threaded cap and the ball have come shooting off the valve. After screwing it shut the first time it happened I noticed it was leaking slightly as well.

My question is how or why would this be happening? Seems to me it would need quite a bit of pressure to pop it open. Or is it just really easy for this valve to be faulty? The valve itself looks is good shape, clean and no visible cracks or damage.

My thought was maybe the rise of the pipe up the hill is too much. I am probably close to 40' below the city sewer line, on a steep incline. Is the valve just trying to hold back to much water after the pump stops pumping? What is the requirement for having multiple valves per elevation change, maybe there should be one and theres not?

Or maybe the valve failing is a symptom of a greater issue? I an irrigation expert, but a residential pluming novice.

Any and all help is greatly appreciated, I'm pulling my hair out here. I could post a pick of the system if it helps!

Sounds to me like you need a new check valve....

If there were a pressure change, the pressure would be greatest WHILE the pump is running, not after it shuts off. There must be a defect in the threads on the valve or in the cap, otherwise it would not come off. You may have lost an "O" ring seal which could cause the leak you describe. A picture of the valve might help, because I have a hard time visualizing how a ball check valve, with a "cleanout lid", would work properly in a vertical line.

Its actually a Y-check valve, installed horizontally before the pipe goes up hill. Looks like this

However there is no O-ring, or at least not any more. Sounds like I'm going to be replacing the valve.

Top quality valve that just needs the missing gasket or o-ring.