In the past several years I have found that the gas tanks in all the cars we have refurbished have been rusty inside on the bottom. It is no wonder when you take two things into consideration, water is heavier than gasoline and the last VW sold in the US is more than 25 years old. The biggest problem this causes is not leaks but the filtering of rust flakes into the fuel line and then into the carburetor. Sometimes these rust flakes are so small as to get past a normal filter and then clump together in the bowl of the carb. I have had many people tell me about buying a new carb just to have the same problems again within weeks. The answer would have been to clean the carb and fuel line and replace the gas tank. There are treatments that can be used in the old tank to combat the rust that often times work well if done correctly. But I would usually rather save my time for something else and just go for a new tank. Replacing the tank is a pretty simple job. Of course it is a good idea to have as little gas in the tank as possible.