New system slowly losing pressure

I have just replaced my home jet pump system and gone to a submersible. My well is 100' deep and the previous footvalve was at 67' and I have put the submersible at 80'. The water level on the day of replacement was 3' below the top of the well head – so lots of water. The submersible is a J series Berkeley, 1/2hp, 3 wire. I have new one inch poly line down the well to the pump. I installed a new pitless adapter, and reused the old one inch line to the house (30' run). I did not install any check valves anywhere in the system, the only one is the one on the pump (plastic). The system was holding pressure initially, but now, two days later, it cuts off at 50 psi and very slowly drops to 30 and the pump kicks in. When I turn off the power, pressure drops to zero.

Cause ? There are no running toilets or leaks in the house. So I am speculating that it is one of 3 problems

1) a bad, or stone in the pump check valve
2) one of my clamp connections is loose (used two clamps at every joint)
3) The old one inch line to the house is leaking

Solution
1) pull the pump and look at the check valve on the pump – if ok install a check valve at the top of the poly pipe just before the pitless. Put everything back and turn on the system
2) If problem persists, dig up 30 foot line to house and replace

Does this sound like a reasonable plan ?? Am I missing something ??

Pull the pump up out of the pitless, put a T with a gauge and valve on the drop pipe then turn the pump on. Close the valve and then turn off the pump, if the pressure drops, the problem is in the well.

Putting a check valve on the pipe at the pitless is a stop gap fix that will create more problems further down the road.

There should only be one check valve at the pump! If it's OK the problem is from the check valve out.