Noticing whisps of steam from the air vents, I diagnosed Heater matrix leak. Added a radiator weld additive, which seemed to cure immediateproblem temporarily. Changed the matrix, bled the system, run car to allow fan to kick in 3 times and then idle for a few minutes. Allowed to cool, opened cap, topped up to max level and took for 50 mile test drive. Once cool, opened cap, everthing OK!. Drove for a week, approx 500 miles. Checked coolant level when cold. Upto top of expansion vessel. Bled system and continued to drive. Checked two days later level OK. So continue to check levels on daily basis, with having to vent pressure from cap before removing. With continued checks, noticed that eventually relieving through radiator cap is evident whilst running. However, no indications from gauges, smell or feel that engine is overheating. (This week stuck in traffic for 2 1/2 hours with no signs of overheating. At the weekend, after 8 hours from turning engine off, when checking fluid level, had to release pressure (coolant cold!). Its got me baffled as the system seems to be working OK. Pulled over the other day, let idle for 5-6 minutes, cooling fan kicked in and didn't restart within the next 5 mins. No evidence of oil in the water and no water in the oil. Head gasket immediate thoughts, but why has the car not seized, or shown signs or smeels associated with overheating. Any suggestions?