Brake light on dash coming on. 2003 Aston Martin DB7 VV

[eaw46]

I live in Canada and have a 2003 DB7 VV with the sport shift wheel. My Brake warning light came on. This is the main Brake system not the E brake light. My warning light does not come on when I first start the car but soon after I start the car it comes on. It does not matter if I hit the brakes as soon as I start the car or not it just comes on about 30 to 40 seconds after starting the car. The brakes work fine I do not have to push the pedal hard or pump them at all they just work as they always have. If I start the car and let it run for a little while then shut it off I can then step on the brake and push the pedal and it will not slowly go down as it would if there was a booster or master cyl leak or check valve trouble. I am flummoxed and cannot seem to find the why in this. Does anyone know a crossover part number for the Booster unit? It seems to be the same as a Volvo or Range Rover of the same year but I was hoping to find a ford one the same. Thanks in advance for any info or advise on this. Car has almost 50K miles and new brake pads front and rear within the last 3 months and brake system was flushed at that time. Eddy