I had good luck with something called "AquaMend" from Home Depot. Basically looks like a round stick of modelling clay, white on the outside and aqua on the inside. You knead it together to mix the two colors, then squeeze it into/onto the crack. Eventually it dries rock hard. I was actually able to fix a crack at the outlet port of my multiport valve using this stuff and it got me thru a whole season, so it definitely holds up quite well.
If it works, take your time checking Ebay for a new multiport. I got a heckuva bargain on my replacement, a lot cheaper than it would have been from an online pool place.
-Chris
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