Dual valve floor pump only pushes air out the Schrader valve hole
You dont cover the open schraeder valve, you loosen the nut holding the presta stem to the bike wheel so you can press the valve stem through to the tire side. When you can feel it through the tire then press on the tire side so that pressure can build inside the valve stem while operating the pump. This causes the auto valve to lock out the schraeder side of the pump head..
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