Paul. The fuel line from the carb to the pump can be draining back depending upon the condition of the check valve in the fuel pump but it’s doubtful that the fuel in the carburetor bowl is being siphoned out as a result.
Which two barrel carb do you have? You said ’57 so I’m assuming a model 2300 Holley. On that model, primary thing to look for would be a leaking power valve that’s draining back into the cavity in the main body behind the power valve. I’d try a new power valve and gasket first. Be sure the gasket is correct for the power valve as there are two styles of gaskets. Of secondary consideration would be the use of a cork gasket behind the metering block and that is allowing fuel slowly weep out the fuel through the gasket from the metering block passage ways. Using a gasket made from the new generation of gasket materials instead of the original cork would fix any 'weeping' issues.