brake fluid level and filler cap

Cap of brake fluid chamberis a diaphragm and a two wire connection. How do your use the diaphagm to check on fluid leved, and what do the two wires connct to-- Is there a warning light somewhere is level gets too low? Any  information on this setup would be appreciated, especially what level to keep fluid in the chamber. Thank you very much.    F L Tabrah Honolulu