1977 Bonanza BRAKE Warning Light ON

[LBJ]

I did a search here for BRAKE LIGHT ON and didn't really find what I was looking for. My 77 Chevy 5/8 Ton Bonanza just had the BRAKE light come ON after I had noticed the brake pedal was going farther to the floor than normal the last time I drove it last week. After the light come on today, I opened the master cylinder and the REAR reservoir was almost empty. Front reservoir was full. I put more fluid in and started the engine and pumped the pedal. The light would go off then on then off, but stayed on after all that. The master cylinder was BRAND NEW 2 years ago, so, kinda think I have a leak somewhere in the rear wheel cylinders or the vacuum brake booster is going bad. I replaced the booster a good 12 years ago and it was a rebuilt one at that, so might be a bad vacuum booster. I looked at the rear brakes for leaks and didn't see anything like a fresh leak, but a previous possible indication at the TOP of the drum backing hubs. I assume the master cylinder rear reservoir goes to the rear brakes, correct? Two years ago I replaced almost all of the brake components such as Master Cylinder, front (rubber) brake lines, calipers, and pads. I don't recall doing anything to the rear brakes. Any ideas on the most likely cause of the problem?