I have a 1976 Mercedes 6.9. The gas tank, in addition to the main fuel outlet on the bottom right above the fuel pump assembly, has two lines that connect to it on the bottom left (drivers side). One that is inboard is larger and I think is the fuel return. That line is clear and when pressurized with air flows to the tank. The second line or outboard line is the question, that line seems to be blocked and will not flow air into the tank. What exactly is that line for ??? Is it suppose to be closed, if so just what does it do ??? I need help here....if I need to replace the Tank because this lin is clogged, or is suppose to be that way.... and maybe only flows out of the Tank...both of these lines were connected to hoses when I removed the tank to clean it.... but when I couldn't blow into the tank I tried to run a wire to unblock it...no success. I need some help and answers on this. DO I NEED A NEW TANK or is it operating properly ?....is it OK. Technical help would be appreciated, Please....

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Stephen