Originally Posted by Psimet2001

Yes the air escaping when you do a decent job of seating and unseating the pump head is the air that is still pressurized and in the pump hose. My implication about technique refers to whether or not the OP is good at taking the head on and off. If not then you can lose a lot of air bumping the tube valve without being sealed anymore.

Release valves on the pump body work by allowing flow of air in the opposite direction - bypassing the check valve in the pump. This allows you to overpressurize and then slowly bleed off air until you reach pressure.

When you reach pressure everything in the system is at the same pressure...that is everything after the check valve:

1. Hose
2. Pump head area
3. Tube Valve
4. Tube

When you kill the seal at the pump head the following happens:

1. Air begins to escape from the hose - leaking around the pump head.
2. Pressure drop in the hose causes the Presta tube valve to close preventing air from the tube from escaping.
3. Air continues to bleed from the head until the hose and head pressures reach stability with the general surroundings - removing the pressure differetial.

When attching the pump head sometimes - depending on your pump's head design - the presta valve is not depressed. This means that you have a 2 chamber system (hose/head vs valve/tube). The valve does not then open until the pressure on the hose/head side is greater than the pressure inside the tube. The pressure differential opens the presta valve. Example...Silca pumps. Pump, pump, pump, pressure jumps then Psss as the valve opens.