We modify this wrench right here in our shop. If a picture is worth a thousand words, what is a video work. Watch the following for a full explanation.
You will need an 8 inch to 12 inch long 3/8 inch drive extension to access the bottom of the pump in the engine compartment. PLEASE NOTE: This wrench will NOT remove the old style "screw-down-to-lock" primer pumps. That require a 19mm wrench or you can use a vise grip to remove. See second video above.