Thanks! That pointed me in the right direction for the radiator hose. Apparently the coolant caps do have a vent that has been known to go bad (as in
this thread). My cap seems like a likely culprit for that issue.
Do you recall if your coolant looked ok for a while, then got brown again? And did it look like mine in the photo above? It seems too brown to me to be just residual, although I confess I've never paid that much attention to it before now.
The dealer said he was ordering some special solvent to remove the oil, although I see on the invoice it just says "soap and water". Maybe a bunch of oil was hanging around in the heater core or something and they didn't do a very thorough job flushing?
Another thought... is the coolant vacuum harming anything, i.e. sucking contaminants into the coolant (like maybe some brown stuff
)?