I would suggest trying some leak trace powder after a thorough cleaning to help you isolate the leak.
The 928 fuel system is quite similar in components and layout to the 924 and 944. I have been building replacement fuel lines for the 911 and 924/944 models for many years.
This is part of what I offer for the front of 924/944 model along the firewall:
For these models the steel hardlines are cut at the floor and Metric compression ring fittings are used with crimped hoses, producing a correct, factory level repair.
A repair like this may be in order for your 928.
Please email me for info. NO pm's.
Len at Autosportengineering dot com