A plunger assembly (300, 500, 620, 700, 820, 1400, 2006) for a vehicle brake system (10, 600, 800, 900, 1000, 1500, 2000), the plunger assembly operable as a pressure source to control brake fluid pressure supplied to one or more wheel brakes (16a, 16b, 16c, 16d, 616a, 616b, 616c, 616d, 816a, 816b, 816c, 816c, 1010, 1012, 1030, 1032), the plunger assembly comprising: a housing (400, 502, 702, 1502, 2128) defining a cylinder having a first port; a reversible motor (422, 522, 722, 1700, 2130) supported by the housing (400, 502, 702, 1502, 2128) and having a rotor (526, 726, 1740, 2740); a linear actuator driven by the motor (422, 522, 722, 1700, 2130), including a ball screw mechanism (420, 520, 720, 1600, 2142) having a screw (1620, 2144) and a nut, one of the screw and the nut defining a rotatable component connected to the motor rotor (526, 726, 1740, 2740), and the other one of the screw and the nut defining a translatable component; an anti-rotation member (1430, 2146) coupled to the translatable component to allow translation and resist rotation of the translatable component within the housing (400, 502, 702, 1502, 2128); a plunger head mounted in the cylinder and driven by the translatable component in first and second opposite directions, the plunger head cooperating with the cylinder to define a first chamber containing brake fluid received from a fluid reservoir, wherein the first chamber is hydraulically connected to the wheel brakes (16a, 16b, 16c, 16d, 616a, 616b, 616c, 616d, 816a, 816b, 816c, 816c, 1010, 1012, 1030, 1032) via the first port, and wherein in at least one operating mode fluid pressure in the first chamber is increased when the plunger head is moved in the first direction and is decreased when the plunger head is moved in the second direction; characterized in that an elastomeric tubular torque coupler (1450, 2148) is provided for supporting the anti-rotation member (1430, 2146) relative to the housing (400, 502, 702, 1502, 2128) for limited rotational movement. a housing (400, 502, 702, 1502, 2128) defining a cylinder having a first port; a reversible motor (422, 522, 722, 1700, 2130) supported by the housing (400, 502, 702, 1502, 2128) and having a rotor (526, 726, 1740, 2740); a linear actuator driven by the motor (422, 522, 722, 1700, 2130), including a ball screw mechanism (420, 520, 720, 1600, 2142) having a screw (1620, 2144) and a nut, one of the screw and the nut defining a rotatable component connected to the motor rotor (526, 726, 1740, 2740), and the other one of the screw and the nut defining a translatable component; an anti-rotation member (1430, 2146) coupled to the translatable component to allow translation and resist rotation of the translatable component within the housing (400, 502, 702, 1502, 2128); a plunger head mounted in the cylinder and driven by the translatable component in first and second opposite directions, the plunger head cooperating with the cylinder to define a first chamber containing brake fluid received from a fluid reservoir, wherein the first chamber is hydraulically connected to the wheel brakes (16a, 16b, 16c, 16d, 616a, 616b, 616c, 616d, 816a, 816b, 816c, 816c, 1010, 1012, 1030, 1032) via the first port, and wherein in at least one operating mode fluid pressure in the first chamber is increased when the plunger head is moved in the first direction and is decreased when the plunger head is moved in the second direction; characterized in that an elastomeric tubular torque coupler (1450, 2148) is provided for supporting the anti-rotation member (1430, 2146) relative to the housing (400, 502, 702, 1502, 2128) for limited rotational movement.