Last week I changed the coolant on the car, as it had never been done. I undid the bottom radiator connection as well as the two heater pipes towards the rear of the car to let the old coolant out. Refilled with correct amount on antifreeze, topped it up as advised on this forum by leaving cap off, running engine, etc. What seems to be happening now, one week on, is that the coolant level in the header tank seems to be dropping by approx 2-3mm per day. Filled it up to maximum line yeterday evening and this morning it had dropped by above amount. I have checked all the places where I removed pipes and they are watertight. I also jacked car up and looked along all pipe runs, as best as I could, and no leaks anywhere that I can see. Is this normal after an antifreeze change - it is a long pipe run on these cars and yet it doesn't have any bleeding places, so wondering if there is some air still in there, as I definitely cannot see any noticeable leaks? Any advice appreciated.