Plenty of pre-units and a bunch of units I have seen have breathers on the timing covers in addition to the crankcase breather window behind the intake cam. Breathers are good, crankcase pressure is high and top end pressure is a fact of life. You might also consider the following: Drill larger holes in the tappet blocks, drill holes and add breather pipes to the rocker box inspection caps (I do one exhaust and one intake on my Bonneville Salt Flats racer). Make sure you have a hose off of the actual crankcase breather stub and vent that pipe to atmosphere or somehwere else that has a bigger capacity (primary or oil tank), but remember on unit bikes the crankcase breather was loopped up around the back fender and vented to atmosphere. If you can get your breather above the engine it works better, because air can get past the oil in the pipe with minimal puke.