Vehicle brake system

The brake system has a service brake unit with a first brake circuit, and a parking brake unit with a second circuit, for wheels (1-6) on an axle. A control device (12) prevents locking of at least one wheel due to a pressure drop in the second circuit, while the vehicle is in motion. The device acts on a shuttle valve (35) with an input, which is connected to an output (32) of the first brake circuit on one side of the vehicle only, and to all outputs (33,34) of the second circuit. The speed of the wheels is measured by a speed sensor (25,26) on at least one wheel, or by two sensors (26,27) on two wheels of the same axle.