inflating presta valves, really dumb question

[baileyj]

alright im totally new to the presta valve (i think that is what they are called, its the skinny one anyway)

when inflating them, how much do you have to unscrew that little metal cap that is on it? everytime i do, i let a ton of air out and then i cant seem to get a good seal from pump to valve? what the hell am i doing wrong?

Thanks, and yeah i know, its a dumb question

 

[pod]

They sure can be a pain. Also known here as a French valve. With a good floor pump there is rarely a problem, unscrew the locking cap all the way, push the pin down a tiny bit to break the seal and let a little air out, then fix the pump piece firmly down over the valve then pump. The larger volume of air and greater pressure in the floor pump easily forces the valve open and shut as you pump.

With a hand pump I find that it is better not to unscrew the cap all the way ( about half to 2/3rds seems to work) especially if the tyre is completely flat in the first place. The smaller volume of air is insufficient to provide enough backpressure to close the valve again after the down stroke. Unscrewing the locking cap just enough to let air in will help it close again on the upstroke. A little trial and error is sometimes required to get the locking cap in the right place. Once there is pressure in the tube the problem usually disappears.

 

[baileyj]

thanks a lot for the help.
Much appreciated