Image Number 4 for United States Patent #6378717.
A closure cap which can screw onto the fixed neck of a cooling system expansion tank. The closure cap is provided with a screw cap and a threaded section in which a combination pressure-vacuum valve is mounted concentrically and in such a way that it can rotate in relation to the screw cap. The valve is provided with a sealing element which comes into tight contact with the fixed neck when the closure is screwed. To endure that the closure cap can be unscrewed only when the excess heat in the cooling system has been completely dissipated, it is proposed that the closure cap when screwed onto the fixed neck would be prevented from unscrewing means of a temperature-dependent control element.