There is a return spring on the clutch that makes it feel like the clutch is still "working". However, what has probably happened to you is a slow leak of hydraulic fluid has progress far enough to prevent the hydraulic clutch from working. Along with that, you may have let the fluid level in the brake fluid reservoir get below the level of the supply line to the clutch system.

So, what you need to do to get going again is bleed the hydraulic clutch system and keep the brake fluid topped off. If it continues to slowly go down, you have a leak in the hydraulic clutch system and will eventually need to replace or rebuild the two cylinders.

This is one problem that might be causing your issue - and I would say it is the most likely.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles