AS a note.
When I purchased my 280SL 16 years ago I noticed that there was a leak behind the insulation
under the dash board at the floor. It must have been there a long time because it rotted a hole in the floor.
Since it is still under restoration , that is the area I will concentration for the leak.
Leakeage from the master rerservior to the line to the transmission.
The back of the instrument cluster is dry, so the only culprit is the master cylinder line to the transmission.
It was a euro car sent to Calif in 1976 . So who knows how long it was leaking.
Tom