heater blower cold+other issues

Hi guys as per my other threads my heat blower has been cold takes ages to warm up I've checked my haynes I cannot find the control valve it is not where it's ment to be and there's nothing else on the pipe work ......... I've heard one pop from behind the glove box when I had a air lock no idea what it is but it's still working and this was after the original issues ...... then just took my cap off because tank looked empty as I did a massive amount of pressure was released and coolent came rushing into the tank like it was unable to get into the tank because of the pressure I've had no fluid loss is it possible I have a air lock in the heater matrix and not in the cooling system and how I.I bleed is it best to leave cap off and let it run?

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