I just bought a Lezyne Sport floor pump. When I screw the schrader valve head onto my schrader valve stem, air flows back through the pump, pushes up the plunger, and quickly all of the air is gone from the tire. When i try to pump air in I can't get above 20 PSI. Is there something wrong with the valve head? Is there a backflow preventer that is malfunctioning? What am I doing wrong?