I've also got the (notso)Smarthead pump that senses schrader vs. presta head. I found that if I don't push the presta valve in far enough it tends to just leak air out as I pump, eventually deflating the tire. I think those pumps work by assuming that a schrader valvestem will only go in about 1/4" inch and a presta will go in more like 3/8" or even more like 1/2".