Originally Posted by
Looigi
Yes. This is the problem. When you take the head off, you invariably lose some air reducing the pressure an unknown amount. The amount of air is inconsequential for a 35 psi huge auto tire, but it is very significant for a high pressure, low volume bike tire.
As stated in my first post, this used to be an issue 40 years ago when the hand pumps for Schrader valves required you to unscrew a hose from the valve and you'd lose some pressure in the process. With the thumblock pump heads that have been common ever since Zefal introduced their HP series pumps this is no longer an issue. Even with 125 psi, 20mm tires the air loss is insignificant - under 1 psi.