When you step on the brakes the light on the dash comes on The fluid is full and the pads and rotors are good?

When you step on the brakes the light on the dash comes on The fluid is full and the pads and rotors are good?
In the XJ6 and earlier (don't know about later cars), there is a pressure sensor which indicates low fluid. When this sensor is bad, or there is air in the line near the sensor, the light will light up when you put your foot on the brakes. Fully bleed the brake system and re-test.