Pressure gathering in fuel tank

[Apolan]

Youll have to excuse my onslaught of tech questions, but im racing against time to finish and MOT my car as i have faculty stuff coming up.

So the problem is that te previous owner has fitted an external fuel pump, which seems to work just fine, albeit for one "small" thing. After a shorter run today the whole tank gets pressurised and i hear the pressure release line hissing under the car, desperattelly trying to get the accumulated pressure out (while also dripping gasoline). If i open the fueltank cap, gasses gush out violently.

So whats the problem? Is the pressure of the fitted pump too high? Should i just increase the size of the outlet/pressure reliefe line? On the end of that line theres a small plastic cointainer and im not sure what it really does other than let fuel/air out VERY slowly.

Thanks,
Alexander