hi Lion,
i think you got the whole concept wrong. the pump itself will not push air out, only water. its the water thats being forced thru the venturi, which cause lower pressure, then the air is being suck in. only the air suck in from venturi and not from the pump.
its the venturi thats contributing to the air.
learn more about the venturi effect here.
http://en.wikipedia.org/wiki/Venturi_effect
if its really not the pump's fault, please clean the venturi thoroughly and totally. not just flushing water etc, use a toothpick/needle to poke thru the holes.
if not, just get a new pump and see how it goes.
all the best.