Our A6 Avant in a hot day makes hissing sound from the tailgate lock, the vacuum boot cracked, here’s how we fixed ours.
Thanks to some A6 enthusiasts who had different experience in fixing this problem. Below is another fix from Chang Liu
I fixed my tailgate just right now following some of your steps. I took out the rubber boot entirely and just used two balloons, some GE silicone caulking, and zip ties. The first try, because my vacuum system is strange, the balloons busted under pressure! The vacuum system in my car goes for about 5 seconds before quitting to ensure the doors are fully locked I guess but it is kind of stupid.
What I did to fix this was use a needle (sowing needle), heat it up on the stove, and then poked about 4-5 holes on the end with the lever arm and the vacuum hose connector. MORE HOLES WILL FAIL. Ask me how I know. To fix it up if there are one too many holes one can use super glue assuming a needle was used to poke the holes. At the end I have my original 5 hole setup and now it works fine.
I’m not planning to keep this car for long because there are one too many problems with it and I haven’t the patience, time, or money to fix all of them. The car is great for now so I will keep using it until I feel like it is time to let go. If it was an A3 TDI I would not let it go but when it gets only 22mpg and has tons of electrical and mechanical issues I don’t have any problems saying goodbye!