I've worked with bikes for 40 years and have used slime for at least 20 years. These things have a GREAT DESIGN!. They have a little red gasket that compresses around the extension pin when you tighten the Schrader valve and doesn't let air through. After several different valve cores, three different air liners, I finally figured it out. I had to rip out part of the red gasket to create an air passage. Not an easy job, the material feels like hard plastic and must have been pressed in.