Originally Posted by
Korina
Basically none, the air's coming from the pump hose.
That is true only for a Presta valve. Prestas are check valves that only allow air flow in one direction. Schrader valves have to be held open to fill. Any hissing is air being lost from the hose
and tube. How much air is lost from the tube would depend on a number of factors and would be difficult to quantify. It would also vary widely.