what does a "check valve" do? Never heard of this part of a pump.
The check valve keeps the air in the tire when you are pumping it.
If the check valve failed, when you pushed down on the pump handle, the back pressure would push the pump handle back up. But because the check valve is there, when you lift the pump handle to put more air into the pump, that check valve is closed and holds the high pressure air in the tube.