I'm running taller then stock valve cover that are chrome with a breather cap on both sides of the engine. I have a TPI setup (speed density) and I'm not using the PCV valve(to plenum) or air port on the passenger side of the throttle body.

So, my valve covers are leaking in the rear on both sides. They are both brand new and made of some sort of teflon or something. Cost like $30 for both of them. Could this be from no PCV valve?? I figured that the oil breather caps would allow enough pressure to be released.

 

Are they leaking oil out of the breathers?
Or are they leaking where the valve cover meets the head?

 

They are leaking from the seal/gasket towards the rear of the head.

 

Has nothing to do with blowby or crankcase venting (although we should talk about not using a PCV system).
Check the valve cover/gasket/mounting hardware. Probably will be the new valve covers mounting surface not being planar (ie: not flat).