Regarding Davidd´s note, that the chain oiler´s tube is too small in diameter for the blowby volume, I would like
to put your attention on the rocker bearing, where the majority of air volume has to pass opposite to oil flow
downwards through the tiny oil channels at its bottom. This area is even smaller than the chain oiler tube thus
resulting in higher air speeds against oil flow direction.
Consequently, any breathing from the top of the valve springs is located at the wrong place, as the highly
accelerated air speed at the rocker bearings will enlarge the oil content in the blowby.
Therefor, any blowby should be conducted from top of the inspection caps over the valve clearance adjustment screws, where air speed is much slower to avoid any hinderance to the oilflow down the rocker channels.
Anybody should note, that non-return valves, no matter, what kind of, do have their individual limit where they
remain open. Therefor, it makes sense - as adviced by PEI in TFS - not to use too large a diameter of breathing
tube, to use the inner resistance of the breather tube to restrict air volume entering the oil tank. He recommended an inner dia. of app. 1/4" for a twin engine in good nick. For normal road use, it might be
sufficient, to run yr blowby return line even without non-return valve, which makes no sense at all on a race
engine.