5.9 engine, 186,000 miles.  If my truck sits for a time, the first time I use the brakes, even the very lightest application you can apply, causes the front brakes to lock up.  I'm talking leaving rubber on the pavement lock up!  If I drive it regularily it doesn't do it.  I found by accident that if I apply/drag the brakes while backing up that it doesn't lock up as bad. (very strange)  In the past when it was "new" I had the the ABS light on and the dealer replaced the sensor/sending unit on the rear axle hub.  I also have found during 2 emergency stops on the highway due to accident(s) in front of me that they lock up at around 15-20 mph during the "panic" stop and the truck then slid about 10-15 ft to a stop.  ABS is suppose to prevent "lock-up" right?  I recently had the front pads replaced (2 times thinking it was the pads) and the brake system flushed last week and although it seems to brake better, it didn't cure the lock-up issue.  In the past I have had to replace both rear wheel cylinders, ( they failed a week apart-leaking) that ABS axle sensor, and the brake booster/master cylinder.  (the master was leaking into the booster and filling it up).  I think something's not right with the ABS as that system is suppose to prevent lock-ups....