Not worth the time to repair
Don't waste your time removing the old primer pump to replace the o-ring, because the one that is defective is internal and can not be removed because of the crimped design of the pump. The o-ring you see at the top of the pump only serves to seal the plunger knob when threading it down. The new one that you will receive when you order will be a different design and much more user friendly, doesn't need to be screwed out or in to use.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan