Not sure which brake light you are talking about....
If it is the one in the dash.....a very likely cause would be that the brake fluid is slightly low in the master cylinder. Even very slightly low will trip the fluid level sensor. You will also want to make sure that you have a firm pedal.......is a very good idea to bleed the brakes, as they get a lot of crud in the fluid at each wheel.
Also, it is a good idea....when doing the brakes, to wash the wheel sensors and the sensor gear. It is normal for a lot of brake dust to build up there.
Another thing to check is the spacing between the sensor and the teeth of the sensor gear.
I have pictures of my rear brake job in my pictures at
http://community.webshots.com/album/201931518cScpNK