countax

The way the fuel system works is the pump provides more than adequate fuel flow for an engine at full power. Excess fuel is returned to the tank. Tank breathing is provided by a small pipe venting at the filler cap when the cap is removed and the tank is being filled. When the fuel cap is correctly installed it seals the tank to eliminate vapours leaking to the atmosphere. The expanding vapours are collected and trapped by the evaporation canister which has an amount of charcoal inside to collect the vapour. When the engine is at 70Deg C and above 1800rpm the purge valve opens and allows the manifold pressure to pull the vapour out of the charcoal canister via a pipe between the canister and inlet manifold. Operation of this purge valve is not allowed on a cold engine to prevent excessive fuel entering the cylinders and messing up the air fuel ratio at idle. There is a 2 way valve between the evaporation canister and tank. I assume this is to prevent liquids moving yet allowing vapour to pass between tank and canister. I suspect this is where you have a fault. the 2 way breather valve may not be allowing air back into the tank once purging has been done, blockage of the canister pipes, one of which is open to the atmosphere to allow the tank to breath, or it could be the purge valve on the canister of wiring at fault.
Hope this helps.
Nigel