rover 416 fuel problem

[keendoitmyself]

i have a rover 416si 1996 a build up of pressure in the petrol tank and then leaking petrol from under the car around near side rear any idea,s

 

[PowerTool]

Are you sure it's pressure and not vacuum ?
i.e. when you remove petrol cap is air rushing out or in?
(could be blocked breather pipe from tank - is petrol leak regular,or only when you release cap?)

 

[keendoitmyself]

Are you sure it's pressure and not vacuum ?
i.e. when you remove petrol cap is air rushing out or in?
(could be blocked breather pipe from tank - is petrol leak regular,or only when you release cap?)

air rushing out petrol leaks before i take cap off then stops leaking