It think the need of a proportioning valve, mentioned many times by others here, should be something to look into. The calipers require more pressure to operate properly than the wheel cylinders on the drums do. Sounds from you description that your front brakes are not doing much work. If you cannot lock up the brakes, they are not doing their full job!
I hope you don't drive this truck on the highway like this. You could be putting yourself and others in danger.
