Honda 225 2006 model

Hi everyone, I have recently had a issue with my Honda 225, the problem first showed itself as about 18 litres or water mixed in with oil, right upto the point where it was oozing out of the dipstick.
When I run water up the water tube, thru the flushing adapter or from any of the 2 thermostat openings with the water tube plugged off I get water rushing out of the vent on the back of the crankcase cover behind where all the connectors are.
Anyone had this issue before?
I've had the oil pan off and it all looks ok.
I think it may be a corrosion hole in the block but can't work out how water is coming out of that vent.
Does anyone know if the mount case has a water passage in it at all apart from where the water goes up thru the water jacket into manifolds?
I have looked at a diagram of the cooling circuit and believe water runs from the watertube up the manifolds, thru heads and block and back thru manifolds on a separate passage than it enters from. Or is there another area I'm missing?I don't want to pull heads off unless I really have to. Leak down test was all good and spark plugs were normal.