Kohler blowing oil into air filter area-- long.

The engine is a single cylinder CV 15 S Kohler and it is about to drive me nuts. History: bought new on a JD
150 hydrostatic mower by a widow lady. Smoked from the beginning and she returned it to the dealer who did
virtually nothing. Dealer sold out to mega store crowd and they pretty much told her they did not sell it so they
didn't care. She took it to a couple of independent guys who did various things but no joy. She got disgusted
and parked it and bought a new Sears mower. After a while she asked me if I wanted it, and of course I did. Well
after putting it back together from the last tinkerer it ran but smoked and after a few minutes began smoking
badly and almost shutting down. I pulled the air filter off and looked into the space and it had large quantity
of oil in it. I did some internet research and found that Kohler had some problems with rings not seating from
the factory. so I removed the head and checked and sure enough the cylinder walls look like they were polished
chrome. I checked for wear and found ~.003, so I honed the cylinder went back with Std rings, lapped the valves
and replaced crank seals and all gaskets plus rebuilt the carburetor with a Kohler kit from John Deere. Put motor
back on mower and it ran fine for a little while then began the same old mess of oil blowing out into breather
tube. I changed the reed on the head for the crankcase vent system because it was standing about .002 of the
seat and book says it should be flat, reassembled and started it up same old story. I then replaced the
cylinder head with one from a running engine that was doing o k. No joy there also. When I did the crankcase
vacuum test as per the Deere svc manual it was showing about 10 inches of pressure on the manometer instead of
the 1-4 of vacuum as prescribed. I did a leakdown test and it seemed to hold ~90% of the air pressure introduced
int the cylinder at 90 PSI and I could not hear any air passing through any where. The crankcase had the right
amount of new 10-30,(Kohler recommended oil) wt H D oil and there is no gas smell indicating gasoline in the oil.
The oil level in the crankcase drops with the running of it. The valve cover fills up and it pushes the oil
through the felt of the breather tube vent oil separator at the top of the cover that is supposed to let the
liquid oil drop back int the valve cover and return to the sump. I am about ready to take the proverbial BFH
treatment to it and be done with it. Anybody have any Ideas?

It sounds like you have done all the usual things to this engine.

Some thoughts that may or may not be worthwhile:

1. Did you verify that there actually was a hole or a passage the oil in the valve cover could drain back to the sump through? I've never seen this problem on a Command engine, but did find that problem on a Kohler Courage Single with just a few hours on it. The other mechanic I worked with also found a similar problem where casting flash almost completely covered the hole in a Kohler Courage twin cylinder engine.

2. did you inspect the area the breather reed mounts to? I recall Kohler had a bulletin for the Command Twins that the casting could be rough in that area and not allowing it to completely seal.

Not a CV15 but may be of help.

https://simpletractors.com/forums/topic/39859-excessive-crankcase-pressure-on-my-kohler-ch18s/

I've have been working on the neighbor lady's John Deere 212 for a few years now and to make a long story sort, the fuel pump keeps leaking fuel into the crankcase. Once the crankcase has plenty of gasoline in it it begins to do exactly what you are describing.
I put 4 fuel pumps on her tractor. Same result.