1973 AMG M35A2 Brake issue

Today I was driving and had to brake pretty hard a couple of times, then noticed that my noticed that my brake pedal was fading and eventually went to the floor. I got off the road and checked the master cylinder and it was full. I ended up taking it apart to see if the plunger was wore out and it appeared to be fairly new. That's as far as I've gotten so far. Anyone have any ideas?

If you haven't done anything to the brake system, I'd bet the rubber hoses have gone bad.

Work the master several times then look under the truck at all lines and at the inner side of the wheels for fluid running out. If it is the master level should go down also.

Line rust through where they are clamped and bolted to frame, is a possibility. There is a common one above the spare tire that is hard to see.

A visual inspection should find a leak.

I would do a full brake service after you find your problem.