brake booster or master cylinder

07 prerunner . On the way to work yesterday after about 2 miles I had to pull off the road. Smelled brakes. checked to make sure e brake was off, it was. 300ft more down the side street I stopped, then could not move the truck. Back wheels spun, but brakes were" locked" in front. Had it towed to local mechanic I trust.
He does not know what it is yet, but said that rotors were blued from heat.
With engine off, front wheels spin, with engine ON, they lock up.
Any ideas? I had a sudden stop the nite before, ABS did its thing. This started after that sudden stop in my driveway. Have not had any problems before. Of note, when brake pedal was released previously( all the time) I hear a noise from the pedal area ( maybe linkage to booster?)
Thanks

 

The following are possible causes:
Brake booster
Master cylinder
Brake caliper
front brake lines

Are BOTH locked up, or just one?

 

A pedal or linkage problem could do it if the booster is being held slighly depressed, and there is also a somewhat slim possibility that the ABS modulator valve is holding pressure to the front brakes. The good news is that if its that repeatable, it should be pretty easy for your mechanic to track down the source - by checking for hydraulic pressure at the calipers and following it up toward the M/C until the culprit is found.