Originally Posted by
Peek the Geek
Was the button located directly on the pump head? On mine (Topeak JoeBlow Sprint) the button is located on the pump body, so I'm not sure how it could depress the presta valve. But I've never attempted to deflate the tire with it. I'll give that a try, for kicks.
I was thinking that the head always depresses the valve. Really don't know.
Originally Posted by
waterrockets
The problem is that you're also measuring the amount of air lost when you replace the pump head. There's no way to know how much of your measurement is from replacement vs. removal. I think a VERY small amount is lost from removal, and almost all of what you're seeing is from replacement.
Also, with my pumps, attaching the head doesn't pressurize the pump since it's at a lower pressure than the tube. So I lose some air getting the thing attached, but I have to pump it a bit to equalize the pressure and get a reading. If you're provides a reading upon connection, then you're pressurizing the pump with air from the tube, which lets air out of the tube. Inconsistencies with this process will also greatly affect the measurements.
The real way to do this would be to set up a rig made from PVC with a pressure gauge attached to it. Then pump it up to 100psi and see what happens to the fixed gauge when you remove the pump.
I defer to your superior method.