My year 2000 Bora 2.0 has since a cam belt change two weeks ago developed back pressure in the petrol tank. I have changed the vacuum valves attached to the black charcoal box but to no avail. If the engine is running whilst the filler cap is off or loose petrol would pour out - petrol being pumped back from the return valve to the outside neck of the filler. Has anyone had a similar problem that can shed light on this for me? Thank you for your time.