Yes, the master cylinder can leak into the brake booster. It can fill up with fluid!

Check all the hoses for signs of blisters in the outer covering. If any are present, replace all four. In fact, if they don't look new, replace all four, they aren't expensive.

Flush the fluid completely while you are at it, too -- unless it looks new, it needs to be changed (every two years!).

If you have no leaks and are not loosing fluid, your used MC is no better than the old original one.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!