If it's only leaking after flushing it's a crack in the rim wash channel(s). Any crack will get worse with heat/cooling expansion/contraction. Eventually it'll grow...even if you use adhesive or sealant. You may be able to stop a growing crack by drilling holes at either end...but makes the risks of making the situation worse kind of high. I'm sure you're following the good/prudent advice of others saying replace replace replace! Good luck.