Crankcase back pressure?

[smbke]

I recently replaced a Kuryakyn Hypercharger with a Chuckster's air kit. There was one vacuum line to the Hypercharger which I plugged and the larger hose going to the reed valves I capped with the supplied plug. I've noticed a marked decrease in engine noise but also a leak from my oil filler cap. The o-ring is new and the cap is tight so my guess is that it's a high pressure leak. Could I be somehow developing back pressure in my crankcase? Would a breather help? I'm really puzzled by this.