The front brakes lock-up when rotors are damp. This only happens when the truck is first started and you come to your first stop. When you gently touch the brakes they start to chatter then lock-up. Then if you pump the brake pedal once hard the problem goes away until the next time the rotors get damp. The truck is 92 F-150 4X4 with RABS. Any help is appreciated.

 

Front brake locking can be either front or rear problems. For the front, make sure the calipers are able to slide within the knuckle. Road salt and rust could have frozen the caliper to the knuckle. For the back, make sure the shoes aren't over adjusted as well as under adjusted. Also make sure that the parking brake strut bar is relaxed and not actually bottomed out on the forward shoe. If it is, you need to de-adjust the park brake cable until there is about .030 clearance between the strut and the shoe. If the bar is not relaxed with the park brake released, the forward shoe is not fully resting on the anchor pin and will cause premature rear wheel lock up on a duo-servo type drum brake.
Phillip