I recently rebuilt top end of my 2009 yz450f. New cylinder, piston, valves, seals, ect. Didn't need to go in to the bottom end, as everything was in spec and fairly fresh. After starting the bike I pulled the oil pressure check bolt on the side of the head and got no weeping oil, so I shut bike down and pulled oil filter , no oil present in the filter cavity. I pulled stator cover off to examine oil pickup screen area, no obstructions. If I turn the motor over by hand (with a drill) it picks up oil and circulates to stator side via the pump. I have blown air through all the oil lines passages, and orifices to try to identify blockage, nothing. If I pour oil through the breather hose and down over the cams and head it fills up in the oil tank. Anyone have any ideas before I tear down any further?''