It takes a long time to get a neglected system un rusted. I believe there's radiator flush detergent products at parts stores that help to clean out the radiator but you'll need to thoroughly clean that stuff out or it would eat your seals, hoses, etc. Heavy amounts of rust indicates your original coolant's rust/corrosion inhibitors have work out and thus your coolant was rusting the inside of everyting steel in the cooling system. water pump, wp bearings, and FREEZE plugs come to mind.. the freeze plugs were likely rusting too. check both sides of your engine block for green/pink coolant deposits or moisture coming from the freeze plugs. I hate mentioning it but also the possibility of your engine consuming coolant is possible by means of an internal headgasket leak. the coolant can leak from the coolant ports into the combustionn chamber and just be burned out the tailpipe. If this is happening you aught to see good amounts of white smoke on a regular basis. Ignore white puffs of smoke upon startup though cause that can be just the water/condensation that builds up in the exhaust. if it burns white smoke after 20 minutes then maybe thats it. Doubt it though. confirmation of burning coolant though is to check the spark plugs for heavy white soot.

oh btw, even a proper functioning radiator will purge coolant into the overflow tank once it's at operating temperatures. That is their design. The cooling system, once cooled off, sucks the coolant back into the system from the overflow tank. The exception to this is when a cooling system leaks and therefore cannot build pressure (when hot) and vacuum (when cold). Then the process of transferring coolant cannot take place via the overflow tank.