Like others do, I vent my primary case to prevent pressurization, and the leaks from the bottom that pressurisation can cause.
I drill a 6mm hole high on the inner primary case (approx below the magneto, and press in a Yamaha DT series plastic 90 degree breather adaptor; cheap, and low profile. Then push a short 6mm hose onto the barbed end, and run down level with frame rail.
If the cases are lapped, the sliding plate modified to carry a seal, and the inner case is assembled unstressed (with the correctly selected spacer between the frame and inner casing), and the oil level and drain screws have been sorted, then the casing can be leak free.
Richard