One of the DCs I bought from a junk yard was leaking antifreeze into the oil. My further inspection showed the sleeves and pistons to be almost brand new and the leak was coming from around one of the sleeve o-rings. I concluded the o-ring groove was not properly prepared for a new o-ring. It was sad that this tractor got junked because of mechanic neglect to properly re-assemble.
We all know the o-ring grooves are hard to inspect and hard to clean. To seal properly, the OD of the groove must be VERY CLEAN AND NOT PITED. We all know that age creates curd and rust pits in these grooves.
I clean and re-clean these groves but pits remain. I find that a thin coating of non-hardening Permatex, applied to the groove OD, is good insurance against rust pits allowing the new o-ring to leak.