Volvo Repair: brakes, proper diagnosis, extra pressure

Question
sometimes when I am driving my 1999 S80 the brake light comes on.  On some occasions when this happens, after pressing the brake pedal, the brakes feel as if they are still applied.  I have to apply extra pressure to the accelerator to get the car to move.  It's like driving with the emergency brake on.  Once the light goes out the car drives fine.  I also noticed that when this problem occurs the speedometer goes to zero.  It stays there until the brake light goes off, then it jumps to whatever speed you are traveling

Answer
I have not seen this before.it could be many things especially on a 99 S80 they were not the best Volvo ever made.they do have ptoblems with the ABS control unit but I have not seen one cause this before but it is possible I supose.it will really need to be looked at for a proper diagnosis.

Rege