* A set of brake hydraulic hoses on a high mileage car is a great idea. Hoses usually get overlooked. A hose aneurism will cause a soft pedal (and a dangerous condition). That condition just isn't that common. I've also seen a hose look ok on the outside and start breaking down on the inside, causing some really wierd braking symptoms.
* What was the issue with the old master cylinder? Was it leaking fluid out the cylinder? Did you check (with the mc removed) for fluid inside the booster?
* Did you manage to crack or disconnect the vacuum supply source to the booster (w/o realizing it) while you were working on the master cylinder?

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ASE Master Tech
Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536