Brakes: How to diagnose?

[jsp]

Hi,

I just acquired an 83 300D Turbo, 220k miles. I was able to get it for about $500, and it is in fair condition, but the brakes are bad.
Symptoms: pedal goes nearly to floor with little pressure, and low braking effect. Pumping the pedal helps just a little. Master cylinder is full with no obvious signs of leaking. Between the m/c and the booster, there are signs of a previous leak (paint stripped from the booster), but there does not seem to be a leak now; perhaps it was fixed previously. I had to drive the car home mosly by downshifting and the e-brake.

At this point I have not yet jacked up the car and removed the wheels, because I would like to have a systematic plan of attack, and perhaps have parts ready, e.g. brake hose, caliper or caliper kit, m/c, fluid, etc.

I know there are other threads about brakes, and the impression that I get is that it can be anything. But can someone give a step-by-step approach so that I don't have to replace the whole system?

Many thanks in advance.

J. S. Park