I sorta hate to admit,
but...along time ago, mine did that.
I somehow drilled out the end of the pin, that the latch catches ( Dont remember how i drilled but may have held the headed end with vise grips or pliers) To open cap I push down taking pressure off the pin and pull it out..usually hold in my teeth while refilling..put it on the bike you will loose it.
Fill your tank insert pin push latch forward press cap down till it catches.. done.. crude but works... and looks like it is still ok..