Recently I've been unable to completely fill my fuel tank. It has a 52 Litre capacity and I would normally put 48 Litres in, this has been reduced to about 30ish Litres. The pump just cuts out after this no matter how slowly I put the petrol in. The fuel gauge will then show the fuel tank as full. As the fuel is used the dial goes down quicker. I’m guessing there is some kind of vacuum as when the petrol cap is loosened you can hear the air escaping. The only way I can get round this problem is to not completely tighten the petrol cap and this stops the vacuum. I’ve had a look at the fuel tank and it hasn’t collapsed.

Any ideas on the cause? Many thanks for any help / advice in advance.