I have a 93 f-350 tow truck, when the truck is loaded with a car and the brakes are applied the front brakes lock up, when unloaded the fronts lock up when applied hard. What could possibly be the problem with the brake system?
Start by checking if there's a height-sensing proportioning valve in back with the brake line(s) going to it. Look for a linkage to the axle housing or leaf spring that is disconnected. They use that valve on most trucks and minivans because there can be such a wide variety of how the load is distributed front-to-rear. Its purpose is to balance the front and rear brakes under varying conditions.