Volvo Repair: brakes, brake failure, booster unit

Question
A few mechanical skills, I have been puttering with 240's since 1998. 140,000 miles on my 1990 240 Sedan. My brakes became soft over the weekend after I steped on them kinda hard. I checked the master cyclinder reservoir and it was quite low. The chamber closest to the firewall was the lowest. I filled up the reservoir and pumped the brakes before I turned on the car. They firmed up. When I started the car the brakes were still soft so I put in more fluid. The brakes where firm.About a minute after driving the brake failure warning light came on. I tapped the brakes and it went off. The light came on again and would only go out when I fully depressed the pedal. I went 2 days without the light coming on. Today driving home from work the light came on again and would not go off. Sometimes the pedel goes almost to the floor, sometimes it is firm. I'm hoping that the brakes only need to be bled. Any help would be appreciated.
Thanks

Answer
Dear Zack,
         You must find where the fluid is going.PERIOD!!
         Chances are that the master cylinder is failing,they do fail.I assume you know if any of the brake lines were leaking...A good way of knowing is to use lots of clean cardboard placed under car when car sits overnight.
         If it is the m/cyl look in booster unit for brake fluid that leaked in.
           Zack,this job sounds like a job for someone who knows how to do this job properly.
          The light is a indication that the system is out of balance...when all of system is proper the light should be reset.
           Zack,pay the money and have the job done properly.
            Good luck.
                       Bilbo