Hi! i am having problems with s2 turbo standard brakes.
I complete recondition work for standard front+rear brakes & remove abs brakes.
I also put new fluids & greenstuff pads. Brakes are properly bleeded.

My problem:

Some times brakes disappear totally, i mean pedal feels solid when i press it but no braking happens.There is no difference is brakes cold or hot. Even sometimes when driving off garage and try immediately brakes. Then sometimes when they do work correctly no need much pressing power and tyres lock up easily and brakes work as them should.
Where is the problem?
Brake master cylinder, Brake booster or that little valve between manifold and brake booster..?
I am pretty sure brake boosting or assistance what ever simply drop off sometimes.