Shallow Well Pump Help with Prime

BACKGROUND: Hey all, this weekend I installed a shallow well pump at my new river property. My grandpa used the well 20 years ago and there seems to be plenty of water. It is about 25 feet deep and it was drilled with a sand point....The night before I hooked up the pump, I manually jacked the 23 foot pipe up and down and water pumped up and shot out...so with that said, it appears we are fine on the water front....

Problem, once the pump hits prime and properly shuts off at 40 psI, you can almost hear all the water flow back down into the well. I am assuming it pulled water since it hit prime? Correct? Can a pump it prime without pulling some water?

Now, I have tried everything from inspecting the 23 well pipe, to taking the foot valve into a pump shop. They said it looked okay, granted it must be 45 years old. So, what am I missing that could result in an instant loss of prime and pressure? My head tells me it must be air leak somewhere, but could a subtle air leak cause such rapid loss in pressure? I measured the water depth on the pipe, and there must be 5 feet of water so that cannot be the issue...PLEASE PLEASE HELP ME, I can only go up on weekends so I am losing tremendous amounts of sleep.

Shallow Well Pump Help with Prime