I believe that the 108 has the eccentric nut adjustment. You need to adjust it so that there are a couple of mm's of play before the clutch mechanism actually starts to move.
You may have a constricted flex line, or your hydraulics may be in poor shape. If the fluid has not been changed regularly it can accumulate moisture which can cause corrosion. This is especially true with the slave cylinder. Either condition could cause the clutch to not engage fully and slip.
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