City Water Fill Crack

[BrianW]

"Black goo" is most likely butyl sealant. That's what mine has. You can get it at NAPA or most auto parts stores, although you might have to buy a whole roll of it. For just this small repair, hit up a local auto body shop and see if they will give you a little bit. (Although having a roll around is handy for lots of things...)

[witoke]

I would guess it's just fatigue. The threaded portion gets the weight of the hose and is also being pulled on when being filled, eventually cracking. You could forestall it with a bit of thickened epoxy as a shoulder for the threads or add a support hook for the hose when you fill it so there is no pressure on the threaded portion. Or carry a spare and figure a break every 10 years on a small, cheap, plastic part is normal maintenance...