Oil & Fuel Fiat 450 Water in Hydraulic Oil?

[farmaboy10]

Any ideas how water might find it's way into gearbox/hydraulic system. Radiator has been loosing water, have discovered worn water pump shaft, soon to be replaced.

 

[Bill74]

I have replaced water pump Fiat 450 tractor. now have water in transmission/hydraulic oil. How much oil do I need to replace oil after draining