I know exactly what you are talking about. I picked uo a Topeak Joe Blow Sport (yellow barrel) last week and, even with a well inflated tire (mtn bike, presta), the gauge reads zero until I start pumping air into the tire. Then it starts reading from that point, as if the gauge is only measuring the air I pumped into the tire, not the air already in the tire from the start. Just from experience, I know the pressure is much more than the gauge is reading.

I wonder if it has something to do with the presta valve since the air being pumped can go in but the valve stops air from coming out, unless I press the valve in with my finger? That wouldn't explain why the gauge starts registering the pressure (from zero) when I start pumping though.

Help! (Please ; )