I have one of 'these' pumps, difficult to attach or release from a Schrader valve.
It's only the latest ***mart purchase that's turned out not worth the price.
I've found that careful adjustment of the 'grabber tightening ring' on the end of the chuck -- tightening the ring just-so as/during cinching the lever, then loosening just as/after releasing -- helps some. And yanking hard if necessary; haven't torn out the inner tube yet. At 120 psi, may be a Presta? I'm having just as much trouble because this pump requires use of the little brass Presta-to-Schrader adapter. Good luck.
Mulling purchasing a pump from a bike shop for about 2x what I paid for this one.
When you put a pressure tester, or an entire pump, onto a tire, the air in the tire is shared between the tire & the device you attach. It'll always read a lower pressure than was in the tire. Even moreso because it's hard to attach and release without losing some air. Only an infinitesimal difference for something as large as an auto tire, but a big difference with a low-volume high-pressure bike tire. You're best off to develop a technique to get the pump off quickly, after putting a little more in if necessary, to end up with a tire that's as hard as you need it without further testing.
Last edited by duffer1960; 06-14-09 at 04:49 AM.