It's one of the most common problems with the Opel Gt. It's an inexpensive thing to correct, but it takes some work in getting to the lines and replacing them. If they are the original lines, then they should have been replaced about 30 years ago and then replace a few more times after that if vinyl tubing is used again. They end up turning brown, cracked, missing, and brittle like so many things that are old. Check out this tread.
http://www.opelgt.com/forums/fuel-ta...nside-car.html
Jerry