Brake problem 86 190e

[Lurch]

Had the master cylinder replaced about 6 months/2,500km ago with brakes working fine until yesterday. Suddenly the pedal goes to the floor with braking power non-existant until the last half inch or so. Car will stop without problem from moderate speed (25 - 35 mph) but I really don't want to tempt fate with higher speeds. From the feel of it, I doubt there is enough grab to lock the wheels and again I don't wish to tempt fate trying, and then finding additional damage from doing so. Brake fluid reservoir is still pretty much full. Pumping the pedal results in no discernable change in feel or reservoir level.

Any ideas on what i should be looking for to zero in on the problem would be appreciated.

TIA