The driver can use the hand valve to apply the trailer brakes. Air flow is illustrated in Figure 44. The tractor-protection valve and relay emergency valve are operated by application air, as explained in the foot-valve application.
Closing the hand valve releases the brakes by closing off application air. 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 may result in loss of brakes! Always set the parking brake.