The Yanmar that we have worked great until it suddenly didn't. The only way to really clean (either properly or without destroying the heat exhanger) is with an ultrasonic cleaner. A radiator shop will be able to do this overnight.

There is no visual inspection that will provide a clue. The water flows through it, it looked "CLEAN", it was inexplicable, but the coating of "stuff' blocked the heat transfer. The ONLY thing that it needed was an ultrasonic cleanning.

I won't bore you with an "all the king's horse, and all the king's men" story of how this cost me a couple weeks of time, vacation, and money -- but its a lesson that I wouldn't wish on anyone.

Anyway, you can live with a little drip -- especially with your externally bolted-on pump (which is tractor motor detail that is simple ;^)))) ).