Dave,
The expansion tank has a rubber bladder which divides it into a water side and air side. The air compresses as the water pressure increases.
You can not blow out the lines by adding more air to this valve. It would only increase the air pressure on that side of the bladder.
You can add the air at any point but generally it is done at a high point. Also be careful as wells and pumps have check valves that will prevent the water from being simply blown out. I have one next to the pump and at the end of the inlet pipe in the lake. This is required to keep the water that is pressurized from simply returning when the pump shuts off.
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