[Tommy]

As I understand a PCV system, vapors in the crankcase are sucked out through the PCV valve when engine vacuum is high and fresh air is sucked in to replace it through the breather. At high power settings when vacuum is low, excess crankcase pressure is vented through the breather. So closing off the breather would help build vacuum in the engine during high vacuum conditions (e.g., idle and cruise). But it would hurt during low vacuum conditions as crankcase pressure could escape only through the small PCV valve. So I don't think closing off the breather is a long term solution. . . . As an aside, many race engines run accessory vacuum pumps to lower crankcase pressure as a performance mod. It seems a bit much to avoid fixing a leaky seal.