It has often been my observation that when bleeding a brake system, the master cylinder will fail on older vehicles.
This is due to the silt build up in the reservoir...a combination of oxydized fluid and rust from the systems internals.
Mercedes seem prone to this too.

VW does this a lot if the vehicle is 6 years old or more.
To stop/reduce this happening I now remove the reservoir and flush it out...refit and fill with clean fluid.

The 'slow' sinking of the pedal is the hardening of the umberella cup seals and then being hard, the washed down debris from the bleeeding score the cup seal lips....some times you can stamp the brakes real hard and quickly, and then get a firm pedal...this confirms bad cup seals.

VW Passat: Rear brake pads, instal/parts...$165.00.
If the car is ABS equipped you need to activate the ABS pump.

However...NEVER push back the fluid by retracting the caliper piston/s with out bleeding the fluid out through the bleeder as the old fluid can and will cause problems in the ABS unit.

Reverse flushing the ABS has killed many systems.


.

.

__________________
[http://languageandgrammar.com/2008/01/14/youve-got-problems-not-issues/ ]

"A liberal is someone who feels they owe a great debt to their fellow man, which debt he proposes to pay off with your money."