The hand valve, or "spike" (Fig. 39) is added so that you can apply the trailer’s brakes independent of the tractor.

The hand valve is typically supplied from primary and secondary circuits and plumbed to a two-way check valve (which is also fed from the foot valve). The double-check valve isolates either the foot valve or the hand valve, depending on which one has the highest application pressure.

Note: Some power units are manufactured without a hand valve.

Independent operation of the trailer brakes has two common uses:

• To couple or uncouple the trailer.
• In the event that the tractor goes into a skid, gentle brake applications using the hand valve may be of some use in trying to straighten out the unit (never apply the hand valve if the trailer goes into a skid).