US Patent 8974011 Tractor hydraulic brake circuit with ABS capability

A vehicle brake system includes a pump, a sump, brake units and left and right manually operated brake valves. The brake system also includes, for each brake unit, a solenoid operated on-off valve having a first port connected to a pilot of one of the brake valves, a second port, a third port connected to one of the brake units, a solenoid and a spring which opposes the solenoid. The brake system also includes, for each brake unit, a solenoid operated proportional valve. Each proportional valve controls communication between the pump, the sump and the second port of a corresponding one of the on-off valves.