service brake system message on 2004 Chevrolet Silverado 1500

brake fluid was flushed. now have a message on start up that says service brake system. message goes away after a few seconds & brakes feel fine. message returns on every start up. Why and how do I get rid of it?

From what I can tell the service brake system message should be displayed only in conjunction with the red brake waring light or the ABS light. You can try confirming the brake fluid level is full and the parking brake switch is working properly. If those check don't lead anywhere check for fault codes in the ABS module using a scan tool would be the next step.
1 more answer
it seems like the sensor is causing the problem check the brake fluid level is it filled to spec check the sensor.