A foot/check valve is a one way valve the allows waster to get suctioned into the pipe but won't allow water to leak back out of the pipe once the pomp stops running, A check valve may be found just after the pump on the suction line while a foot valve's located down in the well point.
As a rule if this valve is bad it will cause you to lose prime and fill the suction line with air.
Do you have to prime the pump after it shuts down and sets for a while?
OMG how do I find a leak??
You're not going to like this answer.
To check for a leak in the suction line or foot valve turn the pump off and watch the gage.
If it falls tom zero and the check valve is OK then the well point must be pulled and the foot valve replaced and the suction line put under pressure and checked for leaks.
I would first check the control box setting to be sure it isn't set too high and the next thing I'd be looking at , (if you loose prime) is the check or foot valve.
Let me know what you hind. Good luck, Tom