Originally Posted by
DaveSSS
If a presta valve is used, the answer is none. When a tire is inflated, the presta valve seals as soon as you stop pumping. The only way to lower the pressure in the tire is to disconnect the pump head and push on the top nut with your finger. The pump head should never contact the top nut with rubber seal at the end. I've forgotten to tighten the nut at the top and ridden 50 miles with no pressure loss. You'll also find that after the nut is loosened, the seal needs to be broken by pushing on the nut to let a little air out. If that's not done, it can take a huge pressure to break the seal.
I am not sure to fully get what you are saying. As soon as you plug the pump's head on the valve, there is a loss of pressure inside the tire (air leaving the tire and going into the pump's hose), it's physics. When you connect the pump on the valve, it opens the ''circuit'' and the valve stays open until it's disconnected.
If you are referring to the air loss when disconnecting the pump's head after inflating the tire, then yes, I'd say there is no loss of pressure (or barely) since the valve closes quickly due to the internal pressure being high.