I had the same leak. Turns out the rubber gasket is just a bit smaller than it should be. It barely covers the sealing surface. It's just undersized enough that if not positioned correctly, the gasket won't seal the surface on one side or the other.
My solution was to cut off the two tabs that hold the gasket into the stamped steel top and more carefully position the rubber gasket onto the reservoir before positioning the top, and re-position the retaining bail. I was careful not to move the gasket as I did this, and it's been sealed ever since.
