[Richard Farrant]

Hi Paul,
I have found some info regarding the Air-Lock filler as fitted to your Morris, and Bedford QL. It is not a pressurised radiator. There is no overflow tube as in some radiators. To quote:
"To prevent dilution of antifreeze caused by careless topping up, an 'Air-Lock' type radiator filler is fitted. This filler incorporates a spring loaded valve in the filler tube which permits steam to escape through relief holes in the cap, no overflow pipe being used. The filler cap must screwed right home to open the air lock valve in the filler."

I had similar problems a few years back on the QL and was not filling the rad more than in the past, but I did find the thermostat was stuck open and wondered if it had been playing up and sticking shut as well. Water would blow out and coolant temp was normal, it would then stop blowing out and run normal until it would happen again for no reason. I changed the thermostat and stripped and cleaned the Air-Lock, did find the adjusted ring depth was not correct, so reset it. Watch that you are not overfilling the rad.

regards, Richard