Bongo Fury

When undoing the cap just now, there was a hiss as it sucked in air, I know this is quite common. Is there a reason the tank needs to be sealed untill the vacuum reaches a point were the cap lets air in.

Here is the pressure relief valve parts in the cap, for now I run without it....it will be safe will it?


As I was filling to the brim with oil, diesel started to pour out the tank vent just inside the filler pipe, so I know the vents clear.

it may be that diesel expands when hot, or shrinks when cooling, and the cap must not leak under normal use,it must let air in when the level in the tank drops,it does all this by the little valve,but if under hard cornering it pours out,or the vacuum is to great then the cap is faulty.or needs cleaning out.