A power brake system (1.1) of a vehicle (2) has a pressure medium source (14) and a pressure-medium-operated primary brake system (30) operable as a service brake and steering brake system. The primary brake system has at least one foot brake valve (36a, 36b) and two wheel brake cylinders (40a, 40b) arranged on both sides on a drive axle (8) and operable independently of one another. A pressure-medium-operated secondary brake system (50), operable independently of the primary brake system (30), has a brake control valve (56) and at least one brake cylinder (62a, 62b. The secondary brake system (50) is electronically controllable and has a brake control valve (56) configured as a solenoid valve. The braking force of the at least one brake cylinder (62a, 62b) can be set by feeding or removing pressure medium to or from the latter via the brake control valve (56).