At what pressue should the drainage valve open in the top of the cap to the expansion tank? ...or how to know it works properly?
I have tested a cap (another car) using a spare header tank and pressurising it with compressed air - the pressure rise will stop at the cap pressure setting. If you don't have a spare header tank, you can probably "in-car" pressurise the tank if you disconnect and plug one of the small hoses from it and connect a compressor to the hose adapter on the tank. However, it is probably not worth the trouble as a new cap costs only about GBP 25.
