My pressure tanks air pocket is replenished by the pump cycle. There's an air control valve on the side of the tank that bleeds out excess air if it's attached float is below the adjusted level. Air in my pressure tank is pumped in whenever the pump cycles on. There's air in the pump lines because a Schrader valve lets air in the well pipe when the pump shuts off and the water settles back to it's natural level in the well. The purpose of the Schrader valve you see my be to let air into the tank when the pump shouts off. If there's not enough air in the tank it may be because the Schrader valve (looks like a tire valve core) is stuck or someone put a cap on it.

The cycle goes like this:
1. Pump shuts off after set pressure is reached
2. Water in well head and lines runs back into the well sucking air with it via the Schrader valve which is on the tank or the pressure switch.
3. Pump starts up when pressure is below set point, and pumps all the air in the well and pipe into the tank. Followed by water until the pressure is reached.
4. A float controlled valve on the tank slowly bleed air (if it's adjusted properly or not broken) to keep the air pocket at the right volume. If this valve is not correctly adjusted it will also bleed a little water.