I'm not sure why they'd run them that way, but usually there are valves on the ends, down at the collectors. The exh gasses create a vaccum as they pass over the end of the tube the valve is screwed into. The vaccum opens the valve, and crankcase pressure and vapors go into the exh that way. If the valve is anywhere else but the collector, the vaccum signal is either not strong enough, or pulses too much, and the system doesnt work. With boost, std PCV systems are not enough, and dont function (boost pressure on the upstream side forces the valve closed..vaccum holds it open..)