Abstract: A braking system for a vehicle such as a trailer having electric brakes which utilizes a carbon-pile voltage regulator. A piston of the regulator is responsive to hydraulic pressures occurring in the tow vehicle brake system to bias the disks to increase current flow. An electromagnet is utilized to preload the carbon disks with a gain control provided in circuit with the electromagnet to vary preloading. Mechanical means are provided in the regulator to limit outward displacement of the piston component of the regulator and, in a modified form of the regulator, a core of the electromagnet. Hydraulic fluid acting on the regulator is isolated from the fluid of the hydraulic brake system by a diaphram unit.