Brakes and Strange Symptoms

I drive a 1994 Oldsmobile 88 Royale. My brakes haven't been working near as well as they were when I first got the car. The symptoms are wierd though. At first, my brake light would come on in my dashboard when I accelerated. It kept coming on for longer and longer periods of time. After a while it would come on an make its dinging noise that it makes. I changed my front brakes and it stopped for awhile. But, after awhile, it started again. After awhile, I changed my front brakes again and it stopped for awhile. But, eventually it started again, but this time it started quicker than it did at first. So, I started to read around on internet forums and I read that there is a sensor in the brake fluid resevoir that tells you when the fluid is getting low. So, I checked the resevoir, and sure enough it was lower than usual. So, I refilled it and the brake light stopped. One problem solved, however, now I had to figure out why the brake resevoir was low. I started thinking about it and came to the conclusion that my rear brakes were going out. This would make sense. More fluid would be going back to hold the rear brake shoes out, when the front brakes got low the resevoir would get below the sensor level. Then when I changed my front brakes, the fluid would get back above the sensor level. Then the next time, the sensor came on quicker because the rear brakes were more worn. This all seemed to be pretty good logic to me. This weekend, I go to change my brake shoes and they're in almost as good of condition as my new shoes. So, now I'm just lost. Anyone have the same problem ever or know what could cause this, any help would be much appreciated.