I changed the sand in my sand filter, and after reassembling, I found I have an extra 2" o-ring. Most of the o-rings in the system are 1.5", and I'm not sure where it came from. The pump is still inside from the winter, so it shouldn't have come from a connection from there. My suspicion is that it somehow goes over the vertical pipe of the lateral assembly to prevent water from bypassing the sand. There is a groove inside the multiport valve where the pipe goes, but the o-ring doesn't fit well (the o-ring is a little big). I did manage to get it in there, but the valve will not fit over the pipe then. Is there usually something that prevents water from going around the outside of the pipe and back into the pool without going through the sand? Also, there doesn't seem to be any mention of a 2" o-ring on my diagram, which further leads me to believe it is meant to be part of the multiport valve. My system is very similar to the one in the diagram at this page.
As a side note, I did notice towards the end of last year that when vacuuming off the floor of the pool that some of the dirt returned immediately into the pool as cloudy water. I checked the spider gasket and it seems to be in decent shape still.
Thanks in advance for any help,