A brake control unit for a vehicle hydraulic braking system is adapted for incorporation between the master cylinder and the rear wheel brakes, with the front wheel brakes being applied by pressure from the master cylinder. The control unit incorporates an outlet connected to the rear wheel brakes, and an inertia-controlled valve, which closes when the deceleration of the vehicle is excessive, to ensure by means of a metering valve that any subsequent increase in pressure applied at the outlet occurs at a rate less than the rate at which the master cylinder pressure increases. The unit also incorporates an override responsive to front wheel brake pressure which is operative, following closure of the inertia-controlled valve and reduction in front wheel brake pressure, to override the effect of the inertia-controlled valve.