With Traction Control

Remove the reservoir tank cap and strainer.
Disconnect the brake fluid level warning switch connector.
Check that there is no continuity between terminals with the switch OFF (float up). See Fig 1.
Use siphon, etc. to take fluid out of the reservoir tack.
Check that there is continuity between terminals with the switch ON (float down). See Fig 1.
Pour the fluid back in the reservoir tank.
If operation is not as specified, replace the switch.

Fig 1: Inspecting Brake Fluid Level Warning Switch (With Traction Control)