I just looked at the parts diagram for that filter and am not familiar with it. I can't remember what was at the bottom of my Hayward to keep the center pipe centered as you add sand, but I think nothing and what you say about sitting on a small nipple makes sense. You can clarify where the leak is for someone else who knows this specific filter. Is the leak between the valve assembly and the tank? Diagram shows a couple O-rings there. Here's the link where I found the diagram.
http://www.poolcenter.com/parts_filt...i_sandtrap.htm
Did you use any silicone grease on those O-rings when you assembled it?
Sorry I couldn't be more help.
Al