I ump up my tires religiously before I ride, like 3 or so times (at least) a week. they're always losing air. the thing is, they're brand new tubes. and it will alternate, front and back. I'm kind of new to presta valves, but I've never had this problem before. I'm using a Presta to Schrader adaptor to inflate, if that means anything. I'm so confused; I've never had such a slow, peristent leak. Any ideas?