Steve,
Similar issue here with the 300# Hayward installed in 97. Only in my case they left out the short lateral that keeps the sand from dumping out when you drain. What a surprise on the first winterization drain. I'd recommend draining the thing, clean the threads with a scrub brush so no sand is in the threads, then goop the cap with silicone seal and put it back on. Make sure the soft seal, whatever the material is, is inside the cap. Oh, and on my problem, I made a short lateral from a piece of PVC pipe and cut slits in the thing. Quicker and a lot less hassle than dealing with Hayward or the company that redid my pool after I inherited the thing with the house. Don't interpret this as a knock against Hayward...I just did an easier fix that worked.
Al