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1978 Mercedes 240D occasional clutch failure

My clutch pedal loses all function after I drive over 45mph. If I wait 10 minutes, it works again. Does anyone know why?

It could be getting too hot under the hood. As was pointed out in another thread, take a jug of water with you, and when it stops, pour it over the clutch master cylinder (on the firewall), and see if that cures it. If it does, then get the clutch fluid changed.

Unless it’s a cable clutch, but I doubt it. If you know which it is, come back and let us know.

I agree with Chaissos.
If this car has a hydraulically-activated clutch, one of the indicators for failure of the clutch master cylinder is loss of pressure when conditions get very hot. And, if the clutch master cylinder of this 33 year old car is the original, it is very likely that the master cylinder needs to be replaced.

If the car utilizes a cable mechanism for clutch activation, I have no suggestions as to the possible cause.