Not fitted yet however, the fire trap side of the breather is left open to breathe into atmosphere.
Can you clarify that? Do you mean that whilst you haven't yet fitted the breather kit, you've let the firetrap pipe vent to atmosphere?
To breathe from both sides of the block, ideally you want to breathe from the firetrap location, the blanked off bit under the exhaust manifold, and have an oil return to the sump itself (below oil level), and then vent to atmosphere or catch tank.