http://www.scag.com/OPManuals/STT/99STTIPL/99STTIPLcomplete.pdf
Make sure your serial number is listed in range of your model. For safety remove negative battery cable & tuck out of the way. Visually study parts that need to be removed for access to fuel tank, take your time, you may need a helper on this project. Be sure to replace items 48-50, bushing, hose fitting, & fuel hose on pages 16 & 17, as you will probably find bushing & fuel hose hard & possibly cracked. Use vaseline on new bushing, then install hose fitting in bushing & then push into fuel tank by hand. You may find that bushing is culprit & source of leak. Let us know how it goes, thanks.