Originally Posted by Juniper
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.
You're very close in your assumption. In actuality, the gauge is measuring the pressure it's seeing at the gauge head. And since the gauge is not attached directly to the valvestem of the tube, it's only seeing pressure in the hose. The hose of course is not pressurised to begin with because of the way presta valves work as you surmised. The valve only opens up when there is more pressure outside than inside. Thus the pressure in the hose must be higher than the pressure inside the tube in order for the valve to open up and for the gauge to begin registering. In order for this to happen, you must start pumping which will increase pressure outside the innertube, open the valve and allow the pressure to equalise. At this point the pressure inside the innertube is equal to the pressure in the hose which the gauge is measuring and thus the gauge will be accurate.