Daf 75 coolant leak

Hi all, l have an issue with my Daf 75 coolant system. When driving its fine no overheating, no signs of oil in header tank but as soon as i switch engine off water gushes out of header tank under pressure. Has anyone had similar issue? Cheers.

Probably the header tank cap shagged…

That was 1st port of call and changed but no difference

Sounds like there’s back pressure in the system the water pump overcomes when the engine is on.

I had something similar years ago, I think it was an air lock, one of the hoses had kinked or something along those lines.