I have a 4 inch stainless MCK pitless in a steel casing. It works by having a big o-ring pressed against the inside of the casing around the exit. The tool rotated from up top pushes a flipper arm against the opposite side of the casing. I did not do my own installation.
I suspect some possible factors:
- possibly not enough force was used to push against the side of the casing, although I would not overdo it. I did not see torque figures suggested. I don't know what the right amount of torque to use would be. They seem to use the SMCK inside part in their SSMCU Pitless Units, so the instructions would probably be the same. https://www.merrillmfg.com/shop/pitless-units-kits-and-adapters/smcu-pitless-unit says
(5) Turn stainless steel screw inside pitless body clockwise
using custom wrench until O-ring seals on inner wall of
casing.
- the casing is too rough and some leak will occur that could expand with time.
- time has deteriorated things. What started as a seep may have eroded with time to a bigger flow.
I would put in a new o-ring. Merrill part number is MCKOR for 4, 5, and 6 inch units. I would use Molykote 111 if making a choice. It is like anti-seize, but is thicker. Try this search term in a search engine:
"Dow Corning 111 Valve Lubricant and Sealant". I understand that the term sealant seems odd in this context, but I am thinking that the thicker material may actually fill some micro imperfections on a longer term basis. I am not sure. Other documentation does not have the word sealant.
Anti-size does the lubricating function. Pretty much the same as Molykote 111 but lower viscosity. It might help things to smooth out quicker. Maybe quicker to apply. That is what the professionals use for this I think.
With the thicker stuff, will it take a little extra time? Maybe. That time would be inconsequential to me.