Abstract: A hydraulic brake fluid level sensing system having a switch housing above a diaphragm which sealingly encloses a fluid reservoir. A float rod guide extends through the diaphragm and a float rod extends through the rod guide and into the switch housing. A float vertically positions the float rod in accordance with brake fluid level. A switch element is positioned by the float rod to bridge a pair of contacts when the fluid level decreases to a specified level. A slot is provided in the rod guide to provide a vent to the fluid reservoir when the diaphragm is moved down the rod guide by differential pressure thereacross resulting from brake fluid loss.