I'm looking for a smaller diameter brass bolt for the bowl-to-tank connection. One hole accepts the new 5/16" bolt just fine, but the other hole is too small. The old hardware is smaller diameter but its really corroded and I would like to replace it.

I can't seem to find a bolt with a large enough flat head on it to get a good seal with the rubber washer. I've tried using a stainless washer on top of the rubber washer but water get in between the bolt head and the steel washer. I could use another rubber washer in that joint, or maybe JB weld the steel interface.... Well you see where this is going.

Any advice where I can get the right size bolt?