Here's some input that may help out:

The mechanical fuel pumps (Pierburg, and others) have small metal flap valves in them that promote pumping gas in one direction when the car is running, however these valves do not provide a positive seal against backflow when the pump is not operating. Having rust particles in the pump (blocks open flap valves), a leaky pump diaphragm (usually rubber distorted due to strange gasoline additives), a pressure/vacuum in the fuel tank (usually a clogged fuel filler cap vent), and/or manifold heat rising onto your fuel line can all affect a fuel backflow problem.

Wasso and Volvo locking gas caps used on 120's all seem to be vented
Later wagons (after '65) had non-vented caps (but have vent tubes in the gas filler neck)

Although I use one on my car too (it looks great), make sure you keep that clear fuel filter routed away from engine heat and impact with metal . . . either one can cause the glass sides to crack, turning it into a minature molotov cocktail!