[DanEC]

If your vent tube at the filler is connected back to gas tank then it doesn't function as an overflow or vent. So you have (or should have) a vented cap. You can verify by carefully cracking the cap open after a drive and listening for air movement due to pressure or vacuum in the tank. If you hear any then the vent in the cap may be closed up.

But from your description of overflow, I would say that a seal on the cap is leaking or you have over-filled the tank - or probably a combination of the two. If fuel is escaping you aren't building up any pressure or vacuum in your tank. I have a Mopar with a side mounted flip top cap (a 67 GTX) that is prone to dripping some fuel if I fill up the tank. I keep the seal coated with dielectric grease which both protects the gasket rubber to keep it soft and helps seal it. Might try that.

Do BDR fuel systems have an internal cap under the flip top - quick fill cap?