Join Date: Jul 2005
Bikes: 1984 Trek 520; 1990s Peugeot (Canadian-made) rigid mountain bike; 2007 Bike Friday NWT; misc others
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There are a couple of possible answers.
In one scenario, the pumps fits a Schrader valve (the same kind as on a car), and you need to use a separate adaptor to fit a Presta valve, which is thinner and has a brass nut at the end you have to loosen before you pump up the tire. If you have an adaptor for Presta, it may be a metal tube with different sized ends. One end goes in the pump and the other slips over the Presta valve. Once you pump up the tire, tighten the brass nut on the valve. Those separate adaptors work well with compressed air hoses but might be difficult or impossible to use with a hand held pump.
Or the pump may have some internal parts that you need to remove and reassemble to fit one or other valve type. Pictures and or brand names would help.