Brake warning light

I have a GMC 1991 S15 4WD. The brakes were replaced last year and truck is used intermittent. Estimated mileage is 3000 annual miles.

The brake warning light flickers on and off when driving. The master cylinder fluid level is OK. The brake petal is OK and braking and stopping is good.

Any suggestions about why the warning light is flashing at me?

My first thought would be to check the parking/emergency brake switch.