I have a 99 SLT, 5.9L that has an intermittent brake issue. Doing a search hasn’t turned up any posts with the same issue. It happens when moving at slow speeds, like moving around in the driveway/parking lot kind of speeds. Hasn’t happened when slowing for stop lights etc. When moving slow and just applying the brakes lightly, the brakes will just suddenly lock up. It doesn’t happen all the time, maybe 20-30% of the time. It feels like it’s the front only, rears are adjusted & lots of meat on the shoes. I replaced the rotors (as one was slightly warped anyway) and both calipers/pads but the problem has come back. There were some slight divots on the rails the pads slide on & I filed them all smooth & greased them when I replaced the calipers. Calipers pins are well greased too. Could it be a master cylinder issue? I’m kind of at a loss on this. TIA for any advice.

I don’t think it’s the same as what you guys are describing. It will do it more than just the first time in the day (its our daily driver) and again always at slow speeds. It does it moving forward or reverse.

The booster was replaced when I first got the truck approx 1 yr ago. The hoses look ok externally, no leakage or signs of cracking, but I guess there could be internal issues with them. I might just replace them anyway if nothing else works. Thanks.