either the cap is faulty or the head gasket is leaking which is common on these,run the bike with the right amount of coolant in the bottle and the cap off and see if it bubbles inside the bottle when its warmed up for 5 minutes if it does bubble then the head gasket is definately gone and should be replaced with a new genuine one ,dont use a pattern one they are rubbish.
adam bike doctor, August 2010
