Hello, Jack from Vegas here. Just purchased an amazingly clean '81 300SD with 190,000 miles on it. Nearly every service record was available, drove it the 300+ miles from Phoenix to Vegas with minimal drama.

There is a few things I want to pick this site's brains on, but right now the most glaring problem occured yesterday.

Coming to a stop the brake pedal went to the floor!!!

Have some mechanical apptitude but have forgotten how to diagnose a power booster or a master cylinder.........

Because it acts like it's not holding line pressure, I'm leaning toward the MC...or should I say I'm hoping it's the MC.

I seem to remember a booster going out on an old truck years back and the pedal was rock hard...

Thanks in advance for all suggestions.