Usually with those type of fittings, you'll have to remove the schrader valve all together.

Like Jason said, don't cheap out on the fittings, get the good ones and they won't leak.

__________________
GreenBastardPerformance2000 GTP Coupe (the green bastard)
XP, Pacesetters, LS1 TB & MAF,MSD System, P&P LIM, P&P GenV w/ 3.2, 3.0, 2.8, Devil'sOwn Injected 100% Methanol,PowerTuned, TripleEdge Trans w/3.29's
12.92 @ 105mph