It is most likely a leak below your check valve below the pitless adapter. It’s 150 ft to water. If the leak is below that, the entire 150 ft could end up filling with air.
Replaced a 2hp pump set 315 ft on galvanized earlier this week. The pump had quit 2 weeks earlier. So they replaced the pressure switch and control box. That bought them a week. Pump went out again on a Saturday. They called another pump company. After waiting 4 days, they called me at 4 pm. Now this well serves three residences and a cattle feedlot with a couple hundred head in it. They were hauling water at this point. Went out and diagnosed the problem The motor had failed. Megger tested bad. No well log. Had a check valve at the surface. Started pulling pipe at 7:30 the next morning. First 80 feet were empty. Static was 30 ft. Had a pinhole leak 10 ft above the first check valve at 105 ft. Dry pipe again. Another pinhole leak down another 80 ft. Replaced pump & motor, drop pipe and wire. Raised casing to above ground level from the 5 foot deep pit it was in, inside a pump house. Replaced plumbing between well head and pressure tank, eliminating check valve. Pressure tested. Held pressure with no change for 1/2 hr. Only check valve now is the one just above The pump. I always add a redundant check valve on a 6” long stainless steel nipple just above the pump. The failed pump was only 6 years old. The previous installer had not replaced the drop pipe or wire. I did not replace the pressure tank. It was a Flexcon FL28 that was 6 years old also. Still at the correct pressure.
so yes, the most likely scenario is a leak in the drop pipe below the top side check valve. Could be any where between the pitless adapter and the pump.
Another possibility, though not very common is entrained gas in the water. I run into this occasionally. There can be a gas entrained into the water. It separates out while setting in the pressure tank. That’s a whole different situation solving that problem, but can be done.
