To change a two- or three-axle unit into a tractor, a tractor system must be added. It consists of the following components:
The trailer-supply valve and tractor protection valve make up the tractor protection system. This system prevents air loss from the tractor when not hooked to a trailer or if a trailer breaks away. The minimum pressure at which the tractor protection system must be activated is 20 psi (140 kPa).