The sensors you need are from RS Components.
Look for the RSF50 Series liquid float level sensor.
They are $21-20 plus GST.
These can be configured to be either normally open or closed by reversing the float on the shaft.
Just drill a mounting hole in the lid of the brake reservoir, fit the sensor and you are done.
They work very well and are the ones I have fitted to the brake and clutch reservoirs in my GT40.
It pays to include the clutch as I had a leak in the slave cylinder seal and if not for the sensor would not have known until I would have run out of clutch and been left stranded.
I got some sensor from Jaycar. My covers on my Reservoirs come off so instead of putting them through the cap I put mounted them in the lid. No floppy wires hanging out of lid.