Low brake fluid.
Possibly due to very low pads?
When was the last Brake Fluid change or last time pads ere checked?

Either start the car, or just switch the ignition on to get the all the dash lights on then carefully unscrew the brake master cylinder cap with the plug on top.
Lift the cap and plug assembly and as it clears the fluid the warning will come on, then lower it back down and the warning should go out. That's how to test the brake fluid level sensor.

Don't spill any fluid as it's nasty stuff and your low anyway. I usually wrap some kitchen paper around he outside of the reservoir before doing this.

 

Mine beeps all the time.
when accelerating, breaking, setting off, stood still.
going to get the fluid topped up shortly.

Devon

 

When on earth the reservoir is in that place I don't know, hate getting the cap off, maybe I just have fat hands lol

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