My water pressure in the house drops rapidly when water is used.
In the past when this would happen there was a valve that would allow me to add air to the tank after emptying it to restore the pressure. Apparently over time the air would be replaced by water and the tank needed to be repressurized. This was a regular water tank without a bladder.

How do I add air to a tank that has a bladder? Where is the valve that air can be added? The pressure in the tank is 60psi. The pump turns on at 40psi. It is a small tank and does not take long to fill to 60psi nor to drop to 40 psi when something is using water. The tank is a Well-/-Troll. There is a red plastic cap on the top... is this the air fill valve (as on a tire)?
I assume I would have to turn off the pump, empty the tank of water, and then pressurize the tank to about 35-40 psi then turn on the pump and let it fill. I do not have a drawing of the tank to show where the air fill valve would be, or if there even is one.