Jadal,
Let me answer some of your questions backward. It depends on the check valve as to weather or not the check valve will hold back the water and what kind of shape the valve is in. If it is a swing check valve it will leak. How much depends on the valve and how it is mounted in the system. If it is a spring check valve it is less likely to leak but it will take head pressure from the pump to operate it. If it has a .5 pound spring in it you will add about 1 ft of head pressure on the pump not counting the additional friction.
When the pump is off there should be no water coming out the primer pot. If you have water coming out then that indicates that the lid is either not tight or the oring seal is not good. It could be a wore out oring or something caught in the oring. It also indicates that the valve on the output or the input is leaking water through the valve. And yes that could flood the pump box depending on how fast water is leaking out the primer pot.
Mike

Thanks Mike. Not sure what kind of check valve it is same kind used on sump pumps. Fixed the priming pot problem have two pumps running with a priming pot on each and had mixed up the lids. The one lid is a little smaller than the other but when switched them out no leak.

Patti,
You have a swing check valve. They are all going to leak some. The good ones will only leak a drop every second to every 4 or 5 seconds. At one one drop per second the valve will let one gallon per day through the valve or not much water. The poorer quality ones will do this when new but quickly get to the point where they let more water back through them. Also a check valve in the vertical position will work better than in the horizonal position.
Mike

Thanks again Mike. Both valves are vertical. You can hear them trying to stay closed or close(?) when pump is shut down. The ones on the sump pumps in the house are really noisy but the one sump pump is pumping 3600gph up six feet and out the side of the house to 40 ft from the house. I just had the one put in and I'm going to bury the pipes and make a rain garden at the end in front yard. In the winter I disconnect the pipes at outlet but sumps don't run in winter at -20F from October to April. The joys of living in Canada eh.