Sounds like there is a steel fitting in the line somewhere forming up its internal coat of rust.

You've discovered the big drawback to pot fillers. They are designed for restaurant situations where they get used multiple times per day not for homeowner use where they might get used once a week. More than just rust will form in the lines if you have hard water. If you are on well water, you can have a pretty good bacteria build up in the still water behind the valve.

Brickeyee - Thank you for the suggestion!

Hollysprings - Not on well water and have tried to use it every day at least once. Hopefully we can resolve.

Hi gardenamy - We renovated our kitchen and went with a Delta pot filler and I am having the same problem. Initially I thought it needed some time to flush out but after a month of doing this, I'm at a loss. Sophie Wheeler isn't onto anything because and I turn on my faucet daily and also not on well water. Regardless, there would have been more complaints from homeowners about pot fillers because they are more common now. Did you ever resolve your issue? Thanks!

bearclare - Thank you for the update! Sorry I didn't see your response in January. We never did fix the problem so I will see if we also can remove and replace!

Follow up several years after the last post on this. I am having the same issue. We switched out the fitting and now pretty much everything is pvc, plastic, flexible etc other than the actual faucet itself. When we replaced the fitting from the recommendation here, it did resolve for awhile (possibly because we had run it so many times in the testing) but now it is back to putting out the brown until it runs clear. Not nearly as dark but definitely there. So my only guess is that it is coming from the water sitting inside the actual pot filler tubes. Very frustrating and inconvenient. I got mine for functionality as my sink is across the kitchen and now I have to run it each time into something and walk it across the kitchen to dump to be able to use it as intended. I have not found a solution yet after scouring the internet several times. Plumber said I will be living with it this way. That makes me super happy.

The post that there was black pipe (ungalvanized pipe used for natural gas) is exactly what I would have guessed. If you are sure it's not coming from the pipes, then its coming from the pot filler. Do you know who made the pot filler? I would replace the pot filler.

Just FYI: I hate everything about pot fillers. Better to design the sink near the stove in the first place.