Question - switching between valve styles on my air pump
I got a new air pump (a Specialized Air Tool floor pump, if the model matters) on Ebay. One of the main reasons I got it was because the nozzle is supposed to be able to fit on both Schrader and Presta valves. My problem is that right now it only seems to work on Schrader valves, but not Prestas. (The nozzle will fit over a Presta valve, but only in the same manner that a Schrader-only nozzle would because Prestas are narrower than Schraders... in other words it isn't a snug fit that could actually fill the tire with air.) The pump didn't come with any instructions, so I'm wondering if there is something I have to do with the nozzle to make it switch to Presta mode. I did notice that there is a plastic ring at the end of the nozzle that can unscrew and come off, but I don't see how that makes the nozzle fitting any different.
So, how do I get my pump to switch to be able to fit onto Presta values also? Does anyone here have experience with these kind of air pumps? What am I supposed to do to make it work?