I have a WR400F and none of the oil is being returned to the frame via the outlet by the gear shift. I’ve loosened off the banjo bolt on the head and this leaks oil, so it looks like the pump is working. Any ideas ? I’ve been told that it’s possible an O ring between the two crankcase halves could have disintegrated and neatly wedged itself somewhere inaccessible. Could I blow it out from the outside and flush the system or is it an engine strip down. Alternatively, I could fill the engine with sulphuric acid and leave it a few days …….….