Hi, see below, I stripped my 749 DP fuel cap filler, so that you can see how it works. There are 2 holes at "3 & 9" in the filler neck. These will allow fuel to vent upwards, and under the fuel cap. There is then a pin hole in the area under the fuel cap, "10 o clock". This matches a hole in the tank to let the fuel run back in to the tank. Under the body of the fuel filler, there is an O ring around the pin hole to ensure that the fuel goes back in to the tank. This ensure that the fuel does not slosh out over the tank. The body of the filler neck that goes in to the tank, has an O ring, and the filler cap has an O ring. All these ensure that the fuel stays in the tank and breather system. Hope this helps. Hope this helps.
