Originally Posted by
recursive
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.
I don't think this will work because each time you connect the pump to the valve, the pressure from the tire will fill up the pump hose loosing a certain amount of psi from the tire. This is not the case when removing the hose after pumping since the pump hose is already full.
To answer the OPs question, IMO it's not an issue. Most of the pressure release comes from the hose, not the valve...