Each fuel tank has to be vented, it's like a closed system. If this is not the case, a vacuum is created which affects the fuel supply and could even cause the fuel tank to implode. For the above reasons, each fuel tank is equipped with a vent, so that the resulting underpressure is immediately filled with air (so that no underpressure can occur). Usually, this vent is located in the fuel cap and is constructed so that air can be drawn in but no fuel can escape.
To ensure proper ventilation of the fuel tank, the fuel tank is equipped with vent connections where air bubbles can form, which eliminate under- or overpressure through the filler neck or an expansion tank. The overpressure is due to the expansion of the fuel. Depending on how the tank is constructed, this overpressure can only be removed by means of vent pipes or an expansion tank must be used. The Volkswagen Vanagon uses two expansion tanks to store excess pressure, which are connected to the fuel tank by hoses.
The pressure expansion line mentioned here connects the upper parts of the fuel tank and is directly connected to the left and right expansion tanks. It is a plastic pipe which, as in the original case, has shrunk on the Y-pieces, replacing the complete pipe. If you still want to repair the hose with a rubber hose instead of a plastic one, the Y-pieces are also available separately (they are listed under the replacement parts tab). Rubber grommets for mounting in the fuel tank are included but are also available separately (see replacement parts tab).
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