If a jar will not hold vacuum or gas pressure, check for the following: cracks in the jar bottom, cracked or missing rubber rings or seals in the lids or a faulty valve stem. Replacement valve stems and a Schrader extractor tool are available from the jars' distributors or bicycle shops. To replace stems, place prongs of extractor over valve stem and turn counter-clockwise until stem is removed. Drop new valve stem (pin-head side up) into valve and turn clockwise until meeting resistance.