Hi,
this connection is the bypass connection. The bypass system prevents that vapor bubbles from the fuel tank through the entire fuel system. The vapor bubbles are pushed back to the fuel tank through this bypass connection, which has a T-piece with the fuel return line.
The new pumps were installed without connection from the bypass to the fuel tank. The bypass connection on the fuel pump was closed blindly with a screw and a copper washer, see
https://www.sls-hh-shop.de/main/en/mercedes-230-280sl-w113-/47-fuel-tank/247000-p--234 and workshop manual 07-11.
Fuel pumps with connected bypass system don't have a gas hole in the overflow valve in the injection pump. Therefore this overflow valve has to be controlled when replacing an injection pump as well an electric fuel pump.
...WRe
