Pneumatic vehicle brake system and control method

A pneumatic brake system for a vehicle, comprising pneumatically actuatable brake cylinders for actuation of wheel brakes, a first brake circuit including a first group of said wheel brakes and a first compressed air reservoir tank for supplying a first reservoir pressure, a second brake circuit including a second group of said wheel brakes and a second compressed air reservoir tank for supplying a second reservoir pressure, said first brake circuit including at least one combination spring-actuated/diaphragm brake cylinder having a spring-actuator part for supplying a parking brake of a wheel of said vehicle and a diaphragm part for supplying a service brake of said wheel, said spring-actuator part being configured to be vented to engage said parking brake in the event of failure of said first brake circuit, an electronic control unit that controls a modulator for admitting air to said spring-actuator part and for venting said spring-actuator part, said control unit being connected to at least one wheel-speed sensor for sensing a wheel speed of said wheel and configured to (1) determine, from said sensed wheel speed, whether said wheel is revolving with a speed that corresponds to a speed of said vehicle and (2) generate an electrical control signal for an electrically actuatable solenoid valve of said modulator to at least one of directly and indirectly effect admission of air into said spring-actuator part when it is determined that said wheel is not revolving with a speed that corresponds to said speed of said vehicle and venting of said spring-actuator part when it is determined that said wheel is revolving at a speed that corresponds to said speed of said vehicle.