Autozone also told me they had a master cylinder for my 65-F-100. When I got there the counterman (and I call him this because I don't consider him to be a parts man) brought me a dual reservior cylinder and not a single like my truck carries. When I told him it was the wrong part, he said it was the right part but that he had to look it up for a 1967 because there computor didn't go back to 1965. So I wouldn't put a lot of faith in the button pushing, question asking countermen (not parts men at Autozone or Advance Auto). But if I were You Jim I would also look for something as simple as replacing the rubber fuel lines attached to the fuel tank selector valve as you could have a defective inner liner on the one hose that may partially block the flow of fuel when switch to that tank. After all you did say 1991 and thats getting a little old for rubber lines to perform correctly. Just a possibility.