Have a 2001, 300M 3.5L 70000 miles. While driving, brakes were normal then suddenly half of the brake system failed. The brake pedal had to be pushed almost to the floor to achieve braking. No warning lights of any kind and no sign of fluid leaks, reservoir was full. Pumping does not achieve pedal pressure. Replaced master cylinder and bled IAW instillation instructions and Haynes repair manual. Problem still persists. The way the car is driven probably never activates the ABS. I'm thinking one of the solenoid valves in the valve block assembly has failed??? Anyone seen this before or have any ideas. Thanks

 

Booster OK? Booster test:
Car off, pump brake 5 or 6 times to deplete any vacuum boost.
Then rest foot on brake pedal, start the engine. Pedal should "fall away" as vacuum boost occurs, if not booster is faulty.

Common issue is to collect moisture in the booster assembly causing it to fail.
Check the " check valve" on the hose end at the booster. One way flow only, trapping vacuum in the booster.