If a leak, more important than the calibrated orifice of the piston occurs between hole « b.» of the detector and the brake units when braking, the pressure in the chamber « c » becomes inferior to the pressure supplied to hole «a ». This unstable pressure will result in the piston (1) being displaced, drawing with it in its stroke the piston-rod (4) which closes the switch circuit, and lights up the warning lamp on the dashboard.