switch unit 23 held open to maintain the alarm 26 inoperative however if there exist some fluid leakage for example in the second hydraulic line l 2 the pressure in the second input hydraulic chamber a 2 cannot be effectively raised even when the brake master cylinder m is actuated and hence the balance piston 9 is subjected to an increased fluid pressure only on the end face of the left hand side stem 9 1 and thus moved to the right together with the right hand side minor piston 12 2 and adjacent annular seal 11 2 until the abutment ring 13 2 on the right hand side stem 9 2 is brought into abutting engagement with the annular shoulder surface of the adjacent differential piston 7 2 simultaneously with this the switch actuator 24 is forced out of the annular groove 22 to close the electric circuit 28 and thus the alarm 26 is actuated the balance piston 9 once placed in its extreme right position remains in that position due to the frictional drag exerted by the annular seals 11 1 and 11 2 upon the respective stems 9 1 and 9 2 even after the brake master cylinder m has been released to exhaust the first input hydraulic chamber a 1 and accordingly the alarm 26 is kept operative until the pressure failure or leakage in the second hydraulic line l 2 is remedied after the failure has been remedied the pressure in both input hydraulic chambers a 1 and a 2 is naturally raised as the brake master cylinder m is actuated so that the right hand side minor piston 12 2 previously moved to.