Outer Limits 4x4 Board

Hi all
i have an 88 GQ TD42, i have recently had problems with coolant coming out through the overflow bottle constantly, i seem to have figured that problem out with a new rad cap, the system seems to build pressure, but the water never seems to flow through my top hose, almost like there is always air in the system. Anyone got any clues as to what i can try next im thinking maybe the thermostat? im not sure if its opening, the thing is the car never ever over heats. its always just sitting below half and it never draws from the overflow

Any answers greatly appreciated

park on a steep hill with the nose pointing upwards then run the motor and top up water , and be sure to have the heater on

is there a correct way of bleeding the system

The TD42 is pretty well setup that you don't have to bleed it, its self bleeding as the radiator is the highest point, just make sure you have the heater on hot when you do it, have the engine off, fill it to max then start it, let it warm up (don't worry is if spills some out during warm up), as soon as the thermostat opens it will suck water down, fill it up again till full and then cap it. that should get you the full amount of coolant into her