There is a check valve ,spring , in the fuel pump which does two things .
1- when the bowl is full it allows the fuel to return to the tank .
2- when You shut the engine down it holds the gas pressure to the carb
It works on the fuel pressure that the carb is rated for if it goes bad the fuel goes back to the tank and air will get into the fuel line and when it gets realy bad the engine will not run right , loss of power ,etc.
Pete
