Oil to the rockers is metered.
Blowing means gas - otherwise it would be 'pumping'.
As I see it there is crankcase pressure being produced from somehwere. That somewhere is limited to compression in the head, there being no other source that can cause it.
You either have gases escaping past the head gasket or past a(the) piston(s). I would suspect that the tractor does not start too well when cold, has been 'ethered' and the top rings are broken. May be a crack in the head, but far less likely.
That is my guess. I'm afraid it will continue to get on your neves unless you actually do something. A bit of a 'catch 22' situation?