[Tun]

With the engine not running, repeatedly press the brake pedal until it goes hard. The reservoir should be at MAX at this point and the light should be on.
Note the light indicates three things;
1 Hand brake on (so make sure it's not for this test).
2 Low fluid in the resevoir.
3 Low pressure in the brake accumulator (brake bomb).
Start the engine, the light should go off within a few seconds but can take longer depending on the efficiency of the pump. If you watch the resevoir the level should fall as the brake accumulator pressurises. The lower the level the more affective the accumulator is i.e. more presses of the brake before it is discharged.
If the level does not fall much it could indicate a problem with the accumulator, with the pump, with the piping or even dirty fluid. There is a gauze fitted the banjo bolt which connects the pump to the base of the brake accumulator which can be prone to clogging up.