For 230 sls and early 250 sls, I believe that when the main tank is really full the little expansion tank fills up too. However it has only a limited capacity of about 500ml.
The excess fuel just escapes through the overflow pipe as it is supposed to.
The 280 sls have a closed system with a return hose to the inlet manifold.
From what I have read, the fuel cap should NOT be ventilated.
I think the 230 sl fuel system breathes through the expansion tank and overflow to the atmosphere.
Bob (Brisbane,Australia)
RHD,1967 early 250 SL, auto