A closed center design used for modulating output pressures up to 3000 PSI. Can be adapted to a variety of hydraulic systems. Power off-braking through limited stored energy in the accumulator(s). MICO also makes a hydraulic over air relay valve. These valves output air pressure in proportion to the hydraulic input pressure. Hydraulic input pressure, from a component such as a master cylinder or hydraulic control valve, sends a pilot signal to the relay valve to moderate air brake pressure.