check valves for fuel line in 1989 190E 2.6

[190E2]

I would like to know where the two check valves need to be installed in the fuel system on my 1989 190E 2.6. My car is really hard to start most of the time because I believe the fuel has leaked back to the tank or someplace else because the check valves are defective. Once the car starts, it runs great, so I don't think its the fuel pump. Thanks.