High Fuel Pressure around 78 PSI?

I replace the fuel pump assembly, and after I replace it I check for fuel pressure when I turn the key to on my fuel pressure gauge goes up to around 78 psi and it drops back down to 0 in less than 10 minutes. I believe I may have connect the three line wrong that run from the gas tank back toward the engine I know one of the line is the fuel supply line and one is the return line and I think one is a vapor line I'm not sure about that but one of line want connect up why?