The house and faucet are 11 years old, never had trouble.
Also: I never closed the lines during winter, as I understand that those round cylinders on top mean they can handle expanding ice?
When I open the faucet, and there is back-pressure, for instance, from a hose that has a nozzle in closed position, this happens:
What I tried:
Turn the hex ring, both ways, without any change.
Do I need a new faucet? Can I repair it myself?
The trick is to re-seat the stem.
I was online shopping for a replacement wall hydrant. One of the reviews mentioned re-seating, so I decided to dismantle and reassemble the hydrant.
It was as simple as loosening the hex nut, and then unscrewing the stem out of the hydrant. Once I had it completely out, I simply screwed it back in, and locked the hex-nut again.
After this, the wall-hydrant works as before, without leaks.
The stem looked roughly like this, when removed:
