I'm with psfred. I just replaced the booster (twice - 2 bad ones) on the 300D with similar symptoms. Mine even started clunking on the 2-1 downshift. I could hear it leak at the booster when shutting the car down by pressing on the IP kill lever. I'm not sure I would risk flexing the brake lines -at least on a W123. I took my master cylinder off- it was leaking anyway and I replaced it.

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)