I agree with Steve, never heard of an "upper rudder seal" except maybe on a boat. Also not sure what you mean with a tankless system that uses a "flushomeder". That said they do make systems with what they call by a number of names including turboflush, etc. that forces water under pressure but it sits in a tank. Generally I try to fix those aggravating types of leaks by coating the sealing surfaces with a silicone pipe dope. If you are talking about tightening down a porcelain tank to the porcelain bowl with a rubber seal with two brass bolts then you need to tighten them carefully a little at a time to avoid cracking or breaking the porcelain. Good luck.