Booster is definitely bad -- shouldn't loose that much vac that fast.

Booster is bolted in from the inside, and you have to take the pushrod off the brake pedal after removing the under dash panels. Not too familiar with the W126, but I believe you have to pull the headlight swtich knob off and remove the 1" nut underneath, the pry the surround off and remove some screws -- ditto on the keyswitch, pull the surround off and remove screws. Also some underneath. You will have to pull the carpet, should be some plastic "nuts" that hold the bottom of the black plastic panel on at the bottom, once those are off it should pull forward and down, then out.

You may only need to remove the black plastic panel and can leave the lower dash panel alone -- try that first, much easier.

Booster is held on by four nuts on the firewall, remove, then unclip the pushrod on the brake pedal, then the two nuts holding the master cylinder on (might want to do those first), then pull master cylinder away from booster and weasel the booster out. No need to remove brake lines from master cylinder, they are long enough and flexible enough to just pull it forward a bit.

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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!