The spring-operated parking brakes of a tractor-trailer are controlled by a novel electro-pneumatic system. Pneumatic valving through which compressed air is conducted is mounted on the tractor chassis and coupled via an electronic circuit to two hand-operated control devices on the dash of the tractor cab, one for the trailer parking brakes only and the other for the tractor parking brakes. The latter device inherently operates the trailer parking brakes via the circuit if they are not already applied, and the circuit incorporates a sequencing function for operating the trailer brakes slightly before the tractor brakes when the tractor parking brake control is operated. Compressed air for the system is obtained via an engine-driven compressor which charges a tank. A pressure transducer monitors tank pressure for the circuit with the circuit maintaining tank pressure through selective loading and unloading of the compressor. Purging of the dryer is coordinated with compressor unloading. The pressure transducer also interacts with the parking brake control devices and the parking brakes such that when the pressure drops below a certain level, the parking brakes are automatically applied. The circuit also gives an alarm before the pressure reaches that level.