Kit of 4 special valves with adjustment.
 
Deflate a tire to the minimum working pressure you normally use. Check this pressure with a reliable pressure gauge. Take a valve and screw the regulator full. Next, screw it into the tire inflation valve and unscrew the regulator until the air stops escaping. That's the exact measurement, now all you have to do is tighten the safety pin. Repeat operation with the rest of the valves.

Now when you want to deflate your tires, all you have to do is thread the new valves to the tire inflation valves. The air outlet will be automatic to the exact pressure you selected. This is done in a few seconds. Take out the new valves and roll.