valve cover gasket

I have a 460 cubic inch engine in an f100 pick up. It has a roller cam, with Moroso valve covers on it. When started up, it will suck the gaskets from under the valve covers in, letting it leak oil everywhere. It has plenty of crankcase ventilation, ( even to the point of removing the crankcase ventilation valves ) to ensure there is not a restriction. I have been a technician for 40 years, and never encountered this problem. Any advice:2cents: would be appreciated. THANKS