I think you're looking at a schematic of the five cylinder system. Every four that I've seen, simply has a tube from the valve cover down to a connection at the intake manifold.
However, you said that you saw a checkvalve at the crankcase. My daughter's five cylinder has the tube, and I was not aware of the check valve, this gives me something to check (no pun intended).
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in