From day one the cap on my radiator overflow tank has leaked from coolant splashing while the tractor is being used. Replaced the cap and didn't fix it so yesterday the Tech came to the house and replaced the tank. One thing he did is remove the rubber seal in the cap and sanded down any plastic mold leftovers so the bottom of the cap was smooth. He then reinserted the rubber seal, put coolant in the tank, held it upside down and nothing leaked so it appears were good.
He mentioned that this process is one thing that appears to have worked many times.
I know this is an ongoing problem with many tractors so I wanted to post this.