You didn't specify the type of pressure tank your system is equipped with. As LL said, a snifter valve is often incorporated when there is a hydro-pneumatic tank, but not for a captive air tank (uses a 'rubber' diaphragm or bladder to separate the air and water chambers).
If your PT is a captive air type, the air pre-charge pressure would be checked with a tire pressure gauge using the Schrader fitting directly on the tank. This is only performed while the pump is shut off and after the water chamber has been completely drained of water. The air pre-charge pressure should be normally 2-4 psi lower than the pressure switch cut-in setting. That 2-4 psi difference will ensure a small quantity of water will continue to remain within the PT when the pump is activated, which should ensure constant water flow to faucets even if there is a short delay before the pump builds sufficient pressure.
If your pump is only building to 20-psi, that is not the result of the PT pre-charge pressure. If your's is a jet pump, check the jet orifice as it could be partially blocked with debris. Another possible reason for low pressure is a hole in the pressure pipe somewhere between the pump and pressure tank.
