Pump the pedal three or four times and hold it. Does it stay in the same position as you hold steady pressure on it, or does the pedal drop further to the floor? When the pedals drops and the fluid level is O.K. it is good indication the master cylinder is leaking internally.
Finding a person at an auto parts store that knows what kind of master cylinder set up you have on your Turbo Buick is harder than replacing the master cylinder assembly. Should you decide to replace it I would recommend going to Carquest, Advance Auto parts, or Autozone and give them this part number “52-9702”. This is the part number for a complete power master unit. It is the whole assembly containing new electric motor, accumulator and master cylinder. Expect to pay around 300 dollars with exchange for this product that is remanufactured by A-1 cardone and comes with a lifetime guarantee. I replaced mine last summer, because I was having the same problem you are having. The part even came with instructions that were better detailed than my GM service manual.
By the way I noticed a huge difference in stopping after replacing this.
Hope this helps.
