Our W140 600 series has probable low brake pedal/master cylinder problems

Has anyone had a master cylinder problem with a 600 series W140. We have replaced our prized 93 600SEL brake hoses, new rotors and pads front and rear, calibers are ok, and fluid flushed several times, all by repair shop. still have soft and low brake pedal. We have ordered and repair shop now has in hand a new master cylinder costing well over $1000. We hope this will fix the low brake pedeal problem.
Any thoughts? thanks. Bill & Monika Mercer

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Bill & Monika Mercer