Try unhooking one end of the connecting rod that goes from the valve lever arm to the axle. Then push up or down on the valve arm, see if air now fills the air spring. Do not change the length of the rod, the ride height is critical for many reasons. See if you have documentation on the ride height specifications. Check with your chassis manufacturer or a decent truck shop that might give you the specs.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI