When you say the return line, are you talking about the rubber line shown below? If so, there would be no o ring for this. That will slide over the nipple on the reservoir and the pump. It is a low pressure line so it most likely doesn't have a clamp either but I would put a screw type clamp on it just to be sure they don't come off.
Otherwise, are you talking about the line that has a threaded end? If so this is the pressure hose and that would have an o-ring.
Can you get some pictures of the hoses that you are looking at if this is not it? Thanks
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Thursday, July 1st, 2021 AT 6:23 PM