I just looked through the cooling system sections of the CD for the 78's. No mention anywhere of a bleed vave.

My 380SL has a vent hole on the front of the thermostat that very occasionally will discharge a small amount of coolant.

The manual says to fill at the tank and then run the engine at varying speeds until fully warmed up, then check the coolant level.

On the Vintage Forum, the advice on the 4.5's is to fill them by removing the upper radiator hose (at the radiator end) and filling the system through the hose.

Hope you find your problem.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe