I recently purchased a VRO kit to rebuild the fuel side. I installed all of the pieces exactly as I removed them. The kit includes a large O-ring that was not installed becasue my VRO pump did not have one. Nor did i use the vaccum o ring as it was too small.(i reused the old one)

435921 -- KIT AY,MOT PSTN 0435921
436095 -- KIT AY,FUEL PISTON 0436095

Does this Large O-ring need to be installed?