Funny I have never heard of this before. I am checking on a used one out of a 1994 968 for you if you are interested. Send me an e-mail and I will get you a price.
Question. When you go to fill up your tank do you ever have a large pressure release when you unscrew the cap. Hear rushing air? If yes then your cap is fine and the tester is full of you know what.
Eric
Eric.Lynch@itt.com