I tend to use a pump for only one type of valve, and since all my bikes are now presta, I've had no problems. In the past, I always had problems when switching from presta to schrader. I wouldn't even think of using my floor pump on a schrader valve. That would be like making instant coffee in my wife's favorite tea pot.... something that is simply not done. When someone comes along with a schrader valve, I pull the small pump from my VW trunk that plugs into the cigarette lighter. I really think switching between two different valve types is just a bad idea. BTW, I make my pump purchasing decisions based on what is being used at the best LBS in my area. They've got one Silca pump with over 8 years of service on it.