Diesels have very minimal intake vacuum since no throttle plate. My Mercedes 300D's have a tube from valve cover top to inside the air filter, with no PCV valve. They have a complicated oil recovery "bottle" w/ drain tube to the oil pan. I expect the crankcase runs slighly pressurized since diesels have more blow-by than a gas engine, especially when worn. I have never seen a Mercedes diesel engine that wasn't gunky around the front seal and air cleaner, and nothing is blacker than diesel oil.