old (70? yrs) house, cast iron waste lines, in a 3rd floor bathroom that is infrequently used, the sink drain is clogged. the tub and toilet seem to be ok. i have taken off the trap and tried cleaning with a hand snake. i have gotten the snake in 4 feet and have taken nothing out. no hair. just some flakes of what look like rusty metal in the water when removing the full trap and draining the water left in the pipe. on the 4th attempt the snake hooked on something at around 3 feet and took a while to get if free. i stopped for the day.
during the rodding water comes back on the snake (like it is creeping up the snake; capillary action?), brown, rust colored. if i plunge the sink, the water is rust colored.
is it possible that the galv pipe or cast iron waste/vent lines have dropped enough rust to clog the drain? what is the likelihood of clearing this with rodding? what else might be going on here?