Philip,
If fuel is not leaking out, air is not "leaking in"! I doubt clamping the filter tubes will do anything. I can only think of three possibles to your "problem".
1. An injector is dirty and leaking after shutdown.
2. The fuel check valve attached to the top of the fuel pump has debris in it allowing pressure to bleed off after shutdown.
3. Hot weather is causing fuel to vaporize (hotsoak, there is a TSB about it) while sitting in the rail on top of a HOT engine. I'm not saying that is OK but, Ford told me to expect slightly longer cranking in hot weather.
EDIT: Philip...these are in reverse order of their probability.
...Ed
