Originally Posted by
FBinNY
Depressing the valve defeats the design of the Presta valve entirely. The beauty of the PV design (when it works) is that it's a simple check valve. Air pressure outside opens the valve and air flows in. Air pressure inside closes the valve, so no air is lost when pumps are removed. These valves were used very effectively for many decades with simple pumps having no thumb lock or internal check valves, ie. Silca hand pumps.
Correct. The tire valve serves as the inflation check valve and prevents air loss in the hose/pump and when the hose is removed. This gets more important for high pressure, low volume tires like bike tires. It's incredibly simple and straightforward.