Pump holds prime but pressure drops to zero when starting

I had a leaky check valve that caused my pump to cycle rapidly. Once I replaced it, a new problem started. After I primed the pump, it shut off and held pressure at 60psi. I opened a valve, and the pump kicked in around 40psi (I have a 40-60 switch), but almost immediately after that the pressure dropped to zero. I can reprime the pump, and sometimes when the pressure drops it can recover and then run through all the zones perfectly (no pressure drops when it goes from zone to zone), but most often the pump can't recover and I have to shut it off and reprime.

I hope someone can help me track down this problem. Thanks in advance!

Busted bladder, too much precharge, or waterlogged tank.

Turns out my brand new check valve was bad. The reason the pressure was dropping to zero was because the valve wouldn't open when the pump turned on. Replaced it with another new one and I no longer have a pressure drop when the pump turns on.