On my '71 144 the radiator cap is on the overflow bottle, not on the radiator. There is a slow leak on one of the hoses, I am just waiting for new hoses to arrive. Anyway, every now and again I have to fill the radiator via the overflow bottle, which is hard considering there is only a very small hose connecting it to the radiator and air and water does not go through it in opposite directions at the same time. I also do not want to release one of the large hoses, as that will spill anti-freeze everywhere. My car gets hot when the water level drops below about half (in the radiator, not the overflow), then I know I need to fill it up again. The radiator design on these cars are hugely overkill, you do not really have to squeeze out the last bit of air (it will work itself out). If you are confident the radiator is over 90% full, then I would suspect a problem with the thermostat. It usually fails in the closed position which causes overheating. From memory it is also possible to insert them the wrong way round (but don't trust my memory, it is Friday afternoon 4:00 here in New Zealand...).