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I have a 12-in Mansfield 4000 series outdoor frost-free faucet (according to this). It's leaking from inside the stem tube (labeled "1") only when water is turned on. Removing the stem from the faucet, I cannot see light from the stem tube end. However, when I carefully dripped water into the stem tube, a little of that water came out from the holes labeled "2". This makes me suspect that there might be a hole (I assume I have to replace the entire stem in this case) or a deteriorated washer inside the stem. Is Prier a new owner of the Mansfield brand? According to them, the stem should never leak this way.

faucet stem close-up faucet stem

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It turns out that an o-ring (400/500 Stem End O-Ring in the diagram below) is completely absent. Since there is no leak when water is turned off, the washers should have been fine. When water is turned on, the stem end will be pushed against the thick copper part, thus closing the holes. With the o-ring missing, water drips through the seam, into the holes, and out from the handle thread.

Mansfield 400 Series Assembly Diagram

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That is a standard washer based faucet. Turn the water off pull the packing temove the shaft at the end there will be a small rubber washer that needs to be replaced. The washers when new are volcano shaped round with a hole and taller in the center. Remove the screw and old flattened washer, put the shaft back in and screw it in then put a few wraps of valve stem packing on the shaft and replace and tighten the nut , turn the water on this will keep it from dripping when turned off or on with a hose on it and is a complete rebuild.

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  • Thanks for the suggestion, but it's not leaking when water is not on, so I interpreted this to be a leak after the volcano washer. Not sure if you see my (very slim) second picture, but that one is showing where water would come out if I carefully drip water INTO the stem tube at the handle end. – Roc W. Jun 17 '20 at 15:06
  • Finish reading what I said to do change both the washer that is what shuts off the water if it is leaking when off the washer needs to be changed but repacking and changing the washer are a good idea when ones I done do the other or before long you will be back for the other part. The volcano washer stops the water if the leak is after that the seat is damaged and a whole new faucet may be needed. – Ed Beal Jun 17 '20 at 19:09
  • I've now changed all the washers from the repair kit 630-7755 to no avail.. Perhaps I may not be understanding you correctly, but there is no leak when water is off. And the test I did with the stem was with the stem removed from the faucet. The leak path during this test does not involve any visible washers. The water goes from the opening at the handle end (labeled "1") to the holes labeled "2". – Roc W. Jun 17 '20 at 20:33
  • That is a frost free it is designed to drain the water when turned off if you have a hose connected. Are you trying to prevent it from being a frost free? – Ed Beal Jun 17 '20 at 21:39
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    Thanks. It turns out to be a missing o-ring. I just added an answer with a diagram. I should have looked for the diagram and read it carefully. Replacing whatever existing washers on the stem with corresponding ones from the repair kit isn't sufficient in this case. I felt kind of dumb.. – Roc W. Jun 18 '20 at 0:38

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