1995 Buick Le Saber Brake Problem

Hello, My 95 Le saber's brake pedal is becomming unstable. Meaning when I apply the brakes sometimes the pedal goes down a short distance and the
brakes are fine. Sometimes it goes down a long ways but still works but seems sponge like. If I pump the brakes once it comes back up to normal. I have had the drums off the rear the pads are good shape, the front disks are good also. The car has 93000 mi on it the front brakes have been replaced about 5000 mi ago. Oh yeah the ABS light stayed on for one trip [about 5 mi] then never came on since, except during the test at startup. The brake fluid is up in the master cylinder to normal also. I have been under the car and see no leaking fluid there. Also , sometimes this happens [drop in pedal] sometimes it dosen't. Cant seem to find anything in the hanes manual or owners manual about this problem. I suspect it is in the ABS controller but dont know much about electronics, but a little. What the problem is, What to do about it, I haven't a clue. Mabey somebody out there who has had this could tell me something about what to do to fix it, Mabey not but its worth a try. Any advice would be appreicated. snipped-for-privacy@comcast.net
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