06-09-15, 09:30 AM
The tank breathes via the filler cap, when closed it should seal to the breather pipe which goes through the tank and exits to a hose on the underside (or funnel for more recent versions). If you're getting bubbling noises it means there's a blockage so you have the risk of fuel starvation if the tank gets sufficiently depressurised, it also increases the chance of water getting in the tank.