Originally Posted by cbjroms
...in the hose which connects the bottom of the radiator to the thermostat housing I have added a T piece, one limb of which is connected to an expansion tank.
What am I missing?
There's your problem, the thermostat housing should be connected to the top connector on the radiator, and the pipe to the expansion tank should connect in as high as possible in the top hose.
Otherwise, any air trapped in the cooling system would have to travel all the way to the bottom of the circuit before it could enter the pipe and escape into the expansion tank and air doesn't like travelling downwards in a liquid.
One way of avoiding trapped air in the first place is to disconnect the top hose from the thermostat housing and fill the system through that hose until coolant comes out of the housing. Also don't forget to pre-fill the heater matrix if you have one fitted.
Finally, the symptoms you describe could
be due to head-gasket failure allowing compression gases to overpressurise the system. Check for 'mayonaise' stuck to the inside of the oil filler, it's not always present but it's a giveaway if it is.
Hope that sorts it.