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Valve cover and crankcase breather hose are off and disconnected but still have pressure building up in the crankcase then very slowly seeps out. Where does the crankcase vent in and out from? Where could it be plugged/clogged at? Why? and how can it be fixed?

The breather port is off of the head cover. Under the head cover there's a breather plate that is effectively a beffle to keep splashing oil from getting pushed out of the breather port. That's it. Take your valve cover off and take a look underneath to verify the breather plate is installed and that it's not blocked off in some way. I believe it's held in place by a single screw. If this is open and pressure is still building up, then the hose must be plugged. This is the ony way for oil to return to the sump from the head top end and must be open to the crank case.

You can go here
http://partsfinder.onlinemicrofiche.com ... ke-CRF450R
and select the Cylinder Head Cover item to see what's in there.

Thank you, that makes sense. Though the only reason I am still concerned is because the case still creates pressure and lacks breathing ability with the valve cover off... how and whereyou does the vent air travel through the cylinder and head? Could it be plugged somewhere in there?