coolant overflow

[rd070707]

Hi guys my hdi has had problems since I had my cambelt and water pump changed 6 months ago. It blew the seal off two header tank caps before we realised the oil in the coolant was due to oil cooler failure.

So changed that and flushed the radiator and its been better since. But it still leaks a tiny amount out the overflow after a long drive.

I think the seal is going on this cap too but the question is why does it keep blowing the seal? Car runs perfectly and has never overheated