At the cottage I drain system in winter & startup in spring. One year
pressure came up to normal pump stopped & then pressure started to drop
right away.
After a little frustration I found my pump has a valve that directions
say open until pump pumping water & then close until flow looks normal?
Well I had closed it all the way.
This was my probem. after opening the valve, pump started to work
properly.
I have no idea what this thing is for but I leave it open all the time &
pump works great.
Bob
"Stormin Mormon" wrote in
:

It does sound like there is not enough air in the expansion tank. One
well I worked on, years ago. There was a device with a bit of a
diaphragm. Aparently, it had an air intake valve. Every time the pump
turned on and off, it would inject a tiny bit of air into the
expansion tank. Of course, these fail after a while. And then the tank
gets water filled.

If there is a tire valve at top of the tank, you could pump some air
in, using an air compressor. Might help. I havn't got much experience
with well tank systems.