Front Brakes Locking On After Car Idles.

The fluid isn't able to return to the master cylinder when there is no pressure applied causing the fluid to get trapped and when it expands from the engine heat it locks your brakes.
Usually too long of a brake actuator rod or over adjusted pedal travel.
The last posts hit on this, and you can prove this is the problem by loosening the master cylinder from the booster while it's locked and it should release.
If it does, then something is holding the master cylinder part way on blocking the fluid return ports inside the master.(usually too long of a rod between the booster and master cylinder plunger)