Why does brake warning light come on sometimes on a 1990 xj6 jag?

check fluid level in brake master cylinder under hood

also check your brake pads.if it comes on when you brake its pads or if it stays on when your cars on its the fluid.but it was doin both of these on my 92 xj6 and i checked both and it was not either.then i found that my wheel bearings were on there way out which causes the wheels to shake which then puts them out of enlinement which messes with the brake sensors and causes the light to come on.so if you ain't solved the problem already then check your bearings by lifting the car up and check the wheels one by one by holding the wheel at 12 and 6 and try to move them.if theres movement the bearings will need changing asap.hope this helps as this was my problem.