People often wonder about their master cylinder. It's easy to eliminate the master cylinder from the suspect list. Just pop off the three brake lines and plug the holes in the MC. If your MC is OK the brake pedal will be rock hard.

The plugs I used came with a new MC I bought, but it turned out the old one was OK.

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1984 300D Turbo 350K 50K WVO (two tank) "Maeby"