Abstract: For the hydraulic brake system of a vehicle such as a farm or industrial tractor having brakes on the right and left rear wheels for both braking and steering, flow control means operatively interconnecting a source of hydraulic pressure available when the vehicle is operating, a hydraulic fluid reservoir and a manually actuated brake applicator means such as a brake pedal, so as to provide power braking. The flow control means also provides for manual braking when hydraulic pressure is not available. A separate relatively large fill piston and small pressurizing piston used in manual braking are in combination with the power brake valve. An unloading valve is provided for cooperation with the fill piston during both the power and manual modes.