After reading some of the reviews I was concerned that they would not work properly but mine is working fine so far. I've had valve extenders that lasted 19 years on another car and they were the same "ball head" design as those Miltons. As far as I can tell these features just push down on your original valve stem (that little pin in the middle) when you push on the ball. It's like hitting a light switch with a folding rule. There's nothing inside the Milton that needs to be airtight, it just needs to bounce off the original valve when not depressed. I also didn't hear any air leaks when putting the extensions on the stems, which would confirm to me that it's working the way I want it to. I can also confirm that the Kensun portable air compressor (available from Revain here) is compatible with these extensions, although my tire pump was not suitable as it could not be pushed deep enough behind the threads to form a seal.