Fuel Caps

[Wayne Pearce]

I have '73 337G normal asp
I still have small amount of anti corrosion stuff leaking from root of wing when Left hand fuel tank is filled (probably happening when fuel expands and leakage washes small amount of anti corrosion stuff down inside of wing)

What I have found so far !
I blow into wing tip vent and can hear the fuel tank expand, I block off vent and it seems to hold pressure, I let it go and it takes quite a while to release all the pressure, obviously it takes a number of puffs to build up this pressure. you can also sense the difference in air flow from pressure in to suck out (brave huh)
The fuel caps have a neoprene cap seal which seems in good order, the caps also have a neoprene red flap valve which seems in good order. This leakage only happens overnight when a/craft is stationary. And the only evidence is a discolouring residue.

Assumtions so far !
The fuel cap vacuum valve seems ok
The fuel cap seal seems ok
The fuel tank seems ok
The vent check valve seems ok
The line from wing tip vent to fuel tank seems ok

Is there a separate overpressure drain line somewhere or is there a tank filler area drain line that I have missed

any suggestions would be appreciated...... best regards.....wayne