Do we have a faulty foot valve?

We have three 5,000 gallon buried steel tanks that collect rainwater from our roof. Each tank has a foot valve about 12 inches from the bottom of the tank. Each tank has a ball valve that shuts off air and water flow to the main outlet pipe (1 1/2" Sched 40 PVC). We have a 1/2 HP shallow well jet pump and a large Flexcon H2 Pro pressure tank. When we primed the pump and began pumping water from one tank, the system worked beautifully! When the tank we were pulling from was nearly empty, we closed the ball valve on the empty tank and opened the ball valve on another tank. At this point, we lost prime on the pump. We attempted to reprime, but after depositing 30 gallons of water, we gave up! We went back to the original tank, primed the line with about 5 1/2 gallons of water and, again, it works beautifully! Does this indicate a defective foot valve? We don't believe we have any faulty pipes as the system was fully pressure tested over 24 hours when it was installed. Would really appreciate any help you can offer!!

Let the pump pressure up the pressure tank and shut off. Then close the ball valve to one storage tank and open the ball valve to the other. If the pressure falls off quickly, then the water is going backwards and out somewhere in the second storage tank. It could be a broken pipe or a bad foot valve.

A bad footvalve would only cause a loss of prime if all the water ran back to the tank and air got in the pump.