Why does my floor pump only fill a presta valve tire after a pop sound? I have had a generic floor pump from an online bike catalog company for years, and have filled many Schrader valve tires with it without any problems.   I have recently started to use Presta valve tires.   When I try and pump air into a Presta valve tire, the air doesn't seem to go into the tire.   It takes me several trys at reseating the pump to get it to work, and every time it does work, there is a distinctive "pop" sound.
How do I get it to work every time?
What I find helps, is: tap the valve post a bit to let out a small blast of air (after loosening the captive nut all the way, of course). That helps unstick things and lowers the pressure inside a bit to make it easier to start pumping air in. Just takes a quick tap. If you don't do that, you may need to pump slightly past the pressure of the tire to actually start pumping air in.

A Schrader valve is a spring-loaded valve that the pump head opens up by pushing down on a pin. That means that once the head is engaged, pressure is equalized between the pump hose and the inner tube of the tire. When you engage the head, some air comes out of the tube into the pump head and hose, and when you work the pump, you're immediately pumping air into the tube/tire.

A Presta valve doesn't have a spring. The valve is kept closed with air pressure, and the captive nut helps ensure nothing bumps the valve open. The pump head generally does not press down to force the valve open, and instead the valve is opened by increasing the pressure in the hose past the pressure inside the tube/tire. You generally have to pump until the pressure in the pump/hose/head matches what's in the tube before any air actually flows into the tube/tire, and sometimes a bit past that.

I suspect that the "pop" you're hearing is when external (hose) pressure overcomes the Presta valve, it opens up, and pressure equalizes suddenly between the pump hose and the tube.