where does my fuel go

I have just gone out to start my 1965 Barracuda. I had it running last week and it was fine. Today it didn't want to start so I opened the hood and notice the fuel filter has very little fuel in it. I pour a little fuel in the carb, the car starts and the filter starts to fill with fuel. My carb was rebuilt, I have a new fuel pump and the filter is new. The car is a Formula S with a stock 273 in it. The car is garage kept and there is no wet spots on the floor from leaking. I assume the fuel is leaking back to the tank, but why? How do I solve this? I would hate to go to a car show and have to put gas in the carb to get it started.
thanks for any ideas.