When testing the tractor protection system, the trailer supply valve must close before the pressure drops below what psi?

It is important for the trailer supply valve to close before the pressure drops below 20 psi to ensure proper functioning of the tractor protection system. Option B, 40 psi, is too high and could result in damage to the system if not closed in time. Option C, 55 psi, and D, 60 psi, are also too high and could potentially cause damage to the system if not closed in time. Option A, 20 psi, is the recommended cutoff point for the valve to close to prevent any malfunction or damage to the system. It is important to pay attention to this psi requirement to ensure safe operation of the tractor and trailer.