I have run 1 pound of citric acid in the cooling system heater fully on for 60 miles or 100km which maybe over several days and after a hose flush followed by 3 distilled water flushes I could not detect any further rust. This however were on 617 and 602 engines which both tended to get on the hot side but that treatment curred both engines. Perhaps your treatment using citric acid wasn't adequate just bringing it up to temp before flushing?