The driver can use the hand valve (21) to apply the trailer brakes. The air flow is illustrated in the next diagram. The tractor protection valve (23) and relay emergency valve (25) are operated by the application air, as was explained in the foot valve application.

Closing the hand valve will release the brakes by closing off application air. The air pressure in the chambers and lines will exhaust, also as explained in the previous foot valve application.

CAUTION: TRAILER BRAKES MUST NOT BE USED TO HOLD A PARKED VEHICLE THAT IS LEFT UNATTENDED. LOSS OF PRESSURE WILL RESULT IN LOSS OF BRAKES! ALWAYS SET THE PARKING BRAKE.