Glacier blue style

Hello everyone, I have recently become a proud x type estate owner and have replaced a weeping radiator. For refilling I've followed the Haynes manual's advice of opening the bleed screw on the water pump and filling until coolant escapes, close bleed screw and continue filling until the MAX level on the expansion bottle is reached. Haynes then say run the engine for 20 seconds, the coolant level will drop and continue filling. My coolant level hasn't dropped at all. I've tried running the engine for a longer period and again the level remains on MAX. The capacity is supposed to be over 10 litres and I've only put just over 5 litres in so far. I assume there can't have been any retained coolant if the radiator was completely removed with both pipes left disconnected for several days? Any ideas as I don't want to run the engine with insufficient coolant or an airlock risking localised overheating damage? many thanks David