My girlfriend was driving home this evening and most of her journey involves sitting in traffic for at least 45mins. Up until today there have been no problems, i.e temp going up, fan kicks in temp goes down and so on...........Tonight however temp went right up into red and no hot air. When i checked the pressure was so great it was forcing the water out of the cap on the filler tank (hense loss of coolant, air in system) First of all what would cause this to happen as prior to this there have been no problems at all. Ive just been trying for two hours to remove the air (ive done this severals time before when i changed the coolant) and i just cant shift it. The front bleed screw has no signs of air comming out but the rear one has loads of air. Ive topped up, bled till no air coming from the rear bleed screw ran engine but the pressure keeps forcing water out of the fillertank - bled rear bleed screw, loads of air, topped up and tried again but i cant shift it ( ive bled and topped up at least a dozen times). Is it possible for air to be getting into the cooling system from somewhere or should i start from scratch and dump all the coolant............. Could my head gasket be gone ??