Does the header/expansion tank on the 307 1.4 hdi pressurise? Mine does not seem to yet the fluid does not boil off. Also although the cap sort of screws down it does not actually seem to go fully tight. By that I mean it seems to get fully engaged but a bit more pressure and it turns and you can then turn it to the nearly tight position again and the same thing happens. It is if it is jumping a thread both being plastic.