Keep or lose the valve cover breather?

[David Schwarz]

I ordered new PCV valves today. I pick the Ford turbo version up tomorrow (P/N EV-127-A). Just in case that doesn't solve my blowby issue, I also ordered a PCV valve for a '96 Toyota Supra turbo (a popular choice on the boards). My question: once I install the new valve, should I keep the breather on? I took it off this morning and replaced it with a sealed cap. The car ran much better, so unmetered air was part of my problem. But with a valve designed for a turbo application, do I need to have a breather?