I agree. I would replace the slave cylinder and the flex hose as a first measure. The slave because that is where the foulest of the water-laden old brake fluid collects, and the hose because they are prone to swell shut after some years of use.

Two more observations:

I had a 124 with a 5-speed for many years. I did not ever notice a change in the clutch play or pedal travel as I drove the car from a cold start to fully warmed.

I fought this exact problem on an XKE. Same basic arrangement. Following experienced advice, replaced slave, hose and master cylinder to eliminate them as problems. Clutch still slipped when engine warmed up. Clutch lining thickness was OK. Was advised that remaining causes were glazing of clutch lining and weak pressure plate which apparently also happens over time. Both can cause slippage as engine/clutch warms.

__________________
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