Originally Posted by
urbanknight
That air is not coming from the tire. It's coming from the hose of the pump. If you understood how the valve worked, you'd know why.
QFT.
I have devised a procedure to figure it out for real though. Repeatedly remove and replace the pump head from the valve stem, taking a measurement each time. Do this at various pressures. After your data is collected, you can calculate the average pressure lost as a function of temperature and beginning pressure.
Easy.