Blow-by is the pressure in the combustion chamber during the power stroke leaking down past the rings and pressurising the sump. This pressure causes the oil in the sump to get forced out the nearest hole (typically the one in the side of the block just above the air con bracket, and the rocker cover).
Normally only oil fumes come out in a healthy engine. That brush would do exactly nothing to stop either the cause or effect of blow-by, get rid of it.
Dave
Having that in there would slow what would of been more than fumes from being blown into the manifild.
Eventually the brush would get gunked up with oily ect ect, and none of the fumes would escape out the breather.
I have seen this before aswell, start her up and see how much fumes, oily spray comes out now it has been removed.