Originally Posted by
prathmann
There's only air pressure to overcome with a Schrader valve as well. The pump head (or hose for some pumps) has a little nub that depresses the spring and opens the Schrader valve. None of your pumping force is doing anything to the valve spring.
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.