fuel tank vent hose on 1988 240 injection..??

On my 88 240 injection ,I find the gas in the fuel tank will evapourate
slowly , means the fuel gauage will drop slowly even the car didn't
move.
I find this venting connection  on the  fuel tank   :
1- a rubber hose runs from the fuel tank neck , the other end  goes out
to the car. (free to atmosphere) , I'm wondering this will make the gas
evapourate.?
2-On the sending unit , there are 3 metal pipe opening , two are for
fuelling , the 3rd is blocked by a rubber cap. 

Is this venting system normal ?

Thanks.

 

The hose at the filler neck should connect to the hose on the top of the
tank (there are various valves etc. to stop fuel leaking out in a
rollover) and then these should be connected to a vapour storage
canister up under one of the front fenders. There then should be hoses
connected to this that are used to draw these vapours into the intake
manifold when the engine is running. This system is quite complicated,
and you should look at another car as an example to get yours fixed.