brake problems..(warning light)

In my '73 satellite, ever since I bought it(2 months or so ago), the parking brake warning light has been on.. I replaced the master cylinder and had a power flush done. So all brand new fluid in there..

What reasons are there that the brake light could be on? I'm thinking possibly it has something to do with a vacuum line or something?

Every once in a while, while I press down on the brake pedal, I'll see the light turn off for a quick second.

The brakes still feel very firm, and I can stop with no problem. Every once in a while though, the pedal will go down a litle further than it should IMO. It neve bottoms out though..

Oh and I have Power Brakes, disk front and drum rear..

All suggestions are helpful! thanks