An improved vacuumizer valve assembly is provided which permits precise, infinite valve adjustment from outside of a vacuumizer apparatus, thus eliminating the need to open the vacuumizer chamber during adjustment procedures. The valve assembly of the invention preferably includes a piston and cylinder assembly operated valve member, with a threadably shiftable adjustment stop coupled with the piston and cylinder assembly. During adjustment procedures, the stop is shifted as necessary to thereby alter the stroke length of the valve assembly and thereby the maximum valve clearance achieved when the valve is opened.