Recently had a faulty check valve replaced on a well pump, and I have had problems regulating the water pressure since. The pressure tank itself is not losing pressure. It has been tested. The pressure in the tank is 35 psi. The pressure switch is set at 40-60 psi. The (I believe it's called) dead head pressure of the well pump is around 90 psi. There are no leaks to the house or in the house. To see what might be happening, I run water until the pump kicks on. I turn the water off and wait for the pump to shut off. After it shuts off at about 60 psi. I turn the power to the pump off. I let the system set without water usage for 6 hours. The pressure dropped from 60 psi to about 52 psi. That drop happened within the first 15 min to 1/2 hour then it stablized at 52 psi and did not move form there for the next 5-1/2 hours. Do you have any suggestions.

When you say there are no leaks in the house, are you certain that water isn't leaking past a toilet flapper valve?

If water is leaking past the flapper valve, then the toilet will fill incrementally to make up for the loss.

Try turning the stops off at the toilets for awhile to see if the drop in pressure persists.