1999 4.6L V8 with cruise control 114k miles

I noticed this on a 25 miles drive home towards end of drive that the truck was braking itself slightly, and I started to hear the noise of brakes. Pulled over and noticed heat coming from rear wheels mostly. I knew there was a problem. Thankfully this started occurring only a few miles from home. Had to pull over a few times to let it cool off, but managed to make it back intact. I noticed the rotors were red hot when I was home.

Brought it to a mechanic and they said it could end up being an $1800 job because they'd start w master cylinder but may end up having to replace every component in system. He said this is signs that the system is "contaminated". Needless to say I didn't OK that job and it's been back home since.

I first tried bleeding the brake system, all 4 wheels. When finished, took it for 3 mile test drive. When I got back, threw water on rotors and I could hear it boiling and saw steam, so wasn't fixed.

I installed a new master cylinder today. Did the bench bleed, then the on-vehicle bleed. Everything seemed back to normal on the 4 mile test drive, but again when I got back home, I threw water on rotors and they were all steaming hot.
Popped the hood and felt heat coming from below the master brake cylinder. Poured water over ABS and brake lines and saw steam coming up as well as the sound of water boiling.

Could a faulty ABS or brake booster be the culprit or is it that the whole system is contaminated? Am I going to have to replace every brake line here? The fluid in the old master cylinder was pretty dark so leads me to believe contamination indeed

 

Never put water on hot rotor they crack been there done that. I would replace the calipers.

 

Are only the rears overheating/smoking? If so, it may simply be the parking brake sticking. Even though you have rear disc brakes, there is a drum brake on each wheel for the parking brake. The parking brake is mechanical only and has a cable running from the pedal assembly to a "Y" in the back that pulls on the mechanism inside each wheel. That pushes two shoes out that contact the inside of the rotor hat. These can stick very badly if rusty and frozen up. Pull the rear rotors off and check. You need to spin the adjuster on the back of the hub to retract the shoes. You can also disconnect the cables from the mechanism at the top on the back of the wheels to help getting the rotors off. Once you get in there, you'll see.

And if that doesn't work, replace the rubber brake lines and possibly the calipers. There may be crap stuck in the lines/calipers that is not allowing the brakes to release.

 

The parking brake hasn't "engaged" since I've owned the truck. Whether the lever is pulled or not, it never put any kind of stopping force on the truck. I live where there's no hills so it hasn't been a problem yet, just haven't got around to fixing it. That is helpful to know the mechanics of the parking brake but all 4 wheels are braking on their own, not just the rear. I will open er up later this week