Every pump has a "check valve" that alows air to flow from the pump barrel into the tube but prevents the air in the tube from flowing back into the pump. These valves are either a ball or plunger in a mating seat or a flapper of some sort. If they get dirty or rusty, you get the symptoms you are experiencing. Remove the valve and clean or replace the ball and it should work fine. The Silca .pdf referrs to the check valve as the "non-return valve".