If you have no other breathers (IE: on your valve covers, or front of manifold?) then all the pumping from the rotation of the crank and up & down movement of the crank & rods will attempt to force it out the (only one) back breather, with an increased air speed because of one opening with the ability to actually carry more
oil vapor. One of the real reasons FE's are hard to keep sealed is because most are opposed to modifying the venting (installing breathers) on the valve covers or elsewhere. There is a lot of
oil being whipped around by the crank & rods and the crankcase pressure has to be relieved by some means, breathers, PCV valve, or the earliest method; a road draft tube.