hi. I have been losing coolant recently. After say a 40 mile round trip, once the engine has cooled i check the level and i have lost about half a litre of coolant from my header tank. I have recently changed my thermostat, however i have checked the joints i disturbed at every possible temperature variant and i can find nothing. I have losened and retightened all joints disturbed and my mate who is a mechanic presure tested it when cold which showed no drop. I have taken into consideration that i would lose a bit, due to the air in the system from the thermostat replacement, it has exceeded the amount i would expect it to go down now. There are no other adverse symptoms apart from, (small amount of greenish residue under oil cap, however the car has infreequent use so i would put this down to condensation.) could anybody give any sugustions??