Will not fill second Gas Tank

Need a little help. I have owned my 1974 246 GTS for about a year. It had an engine out rebuild several years back with new replacement fuel lines. My passenger side fuel tank will not fill with fuel. I think it has to be the fuel balancing pipe between the two tanks or a clogged air vent line? The vent line coming off the top of the gas tank is routed back to the fuel filler line. It is a CAVIS clear line as seen in my photo below. Any thoughts would be appreciated!

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I forgot to add this is a left Hand drive Euro Dino 246 GTS