In order for the road draft tube (or PCV valve) to vent the crankcase properly there must be a way for air to get into the engine. At speeds above 30 MPH the air passing the end of the draft tube creates a vacuum to create a flow through the crankcase, and air from the vent(s) flows into the engine. If the valve cover doesn't have slots, you need a vented filler cap or some other way to get air in.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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