I had the same problem with my ped. GO to any motorcycle shop and buy an acid bath kit. Its a 3 stage kit that step one is acid. It burns out the rust and when u empty it the tank looks like new. The 2nd stage is a cleaner. Gets ride of any water/acid left in the tank. And three is a coating that lines the tank with a plasic like layer to prevent rust from forming again. I got mine for about 30 bucks.
Hope this helps,