The line from the valve cover is a metered orifice that Mitsubishi used in place of a PCV valve in 1979 and 1980. That can become plugged in the valve cover - the line is to allow engine vacuum to suck fumes from the valve cover into the engine to prevent pressure buildup in the valve area. The manual suggests you to use a stiff wire or small drill bit to clear the orifice. The other lines may be part of the EGR / Sub EGR system, and it is probably carboned up as well by now - time to pull the EGR and see if the ports are clogged with carbon - very common on these trucks.