The typical way to find air intake leaks is to direct propane or spray water carefully on each intake rubber of the throttle bodies while the engine is idling. If the revs change, there's a leak. Carb cleaner or volatile brake cleaner can be used too, but being liquids, they possibly can pool then ignite so pose greater fire danger that a vapor like propane. Be careful.
Are you referring to the vacuum hose attached to the fuel pressure regulator which controls pump pressure to the fuel rail?
Laitch - You know I'm not sure now - I think I might be getting mixed up with the overflow pipes x 2 that run out of the rear of the tank and drop to the ground.
I also suspect there may be globules of water in the fuel tank.
The seal on the filler cap allows rainwater to build up around the filling hole.
So it's possible that these droplets have found their way into the fuel system.
On my K100 - it's all about the rubber parts.