Mercedes Benz repair: fuel leak, side firewall, 500sec

Question
i have a 1984 500sec just changed the fuel pump car started fine but after sitting for a while the fuel leaked back to the tank i had to put gas in the carb to get it to crank what could cause the fuel to leak back?

Answer
was it a new fuel pum[p or used.chances are the fuel pump is shotty.only way a pump can suck it back is by a siphon system otherwise pumps only pump up hence why the pump is lower than your engine. jump your fuel pressure relay 87 and 30 box on your passenger side firewall should be a 12 pim connector