A control valve is provided for controlling the distribution of pumped hydrodynamic fluid to the trailer brakes of a tractor and trailer combination under the influence of hydrostatic pressure applied from the tractor braking system. The valve has an inlet (80) for pumped fluid, a spool valve (4) operable in response to a control piston (15) to control the flow of pumped fluid to a first working circuit through an outlet (17) and to tank through an outlet (18). A flow control valve (5) provides a passage communicating with a first flow path (22) for pumped fluid to the spool valve and is operable in response to pumped fluid pressure, to control the flow of pumped fluid to a second working circuit through an outlet (19). A second flow path (6D, 27) for pumped fluid extends between the inlet (8) and spool valve (4) independent from and parallel with the first flow path (22) and the spool valve (4) controls the flow of pumped fluid along the second flow path to the fluid outlet (17).