New pressure tank andswitch-no visible check valve

When pressure drops even though no water is being used, it points to a leak. The question, of course, is where? Do you have a shut off valve to turn off water to the house? If so, turn it off. If pressure continues to drop, then the problem is in the well. If not, then the problem is towards the house.

Could be a leaking checkvalve. You can be sure you have one. Is your pump above ground or in the well?

Great advice by Jlisenbe. OK! You've shut the water off to the house and the pressure still drops. What to check next? The most likely culprit would be a faulty check/foot valve . If the check/foot valve's OK the next place to check is the suction line for a leak. Good luck, Tom