Who let The Dawg out...
Awesome Mack. You gotta love this type of mod!
Truck vs. Tractor braking. Trucks have a SBM (Spring Brake Modulator) Inversion valve. Given most trucks don't pull a trailer, the brake system has to function incase the primary (Drive) brake system fails. Tractors rely on the trailer to help stop the vehicle if this condition occurs, therefore not requiring the SBM.
If the primary fails, this valve relies on the secondary (Steer) for air supply, and uses the spring (park) brakes to slow/stop the vehicle. It keeps the spring brake charged, then depending how much the brake pedal is depressed in the cab, proportionally releases air out of the chamber so the spring actuates the foundations. When the pedal is released, air is applied caging the spring, allowing the truck to roll for a few brake applications. Of course, the intent is to allow enough air to move the chassis to the side of the road.
When shops convert tractors into a straight truck (mostly dumps), it's imperative this brake system be adapted. If not, when the primary fails, the spring brakes will lock up the drive axle/axles. Some OEM's utilize spring brakes on one axle (tandem tractor). Therefore with one axle, it's imperative that a second set of spring brakes be added, which is STD on all trucks.
FMVSS 121 requires a fully truck to park on a 20% grade up/down for five minutes without moving. If lift axles are installed, they must be raised during this event, therefore the generally used 30/30 cans be changed to 30/36.
If purchasing a converted tractor (check the VIN#), you might verify it meets federal requirements. ESP/ESC is required on most all tractors, not trucks. If this system remains on the modified truck, it won't meet the design criteria.