Have you tried to use some silicone to stop the leaking. Should do the trick. Had to do it on my 1 gal waterer because the surface wasn't flat where I threaded the nipples in.
We built this waterer this weekend and we didn't have quite the luck others did. We grabbed 3/4 schedule 40 PVC pipe not realizing the original design used thin walled PVC pipe and we had a devil of a time threading the nipples in. We have 7 other nipple waterers, both horizontal and vertical and none of them leak, but every last nipple on this waterer leaked.
We had plenty of other - user caused trouble. All in all, I love the design, if I could stop making mistakes and get it right. In the meantime, my broilers are getting wet heads when they drink.