Exhaust Vacume question

I installed the Morroso exhaust valves into my headers and ran the hoses to my valve cover.I thought the whole purpose was to create vacume so I have a closed cover.A fellow racer said I need a breather in the cover to allow air in,,,thought that I was trying to create vacume??I think it should bee sealed so it would suck crankcase pressure.Thoughts and guidence appreciated.Guzzi Mark