To Wayne C - you asked about a"pancake" brake - it's a compact disc brake assembly where the splined friction plate is sandwiched between two stationary smooth discs. One fixed, one sliding (applied by a spring and pulled back by a solenoid). Commonly mounted on the end of a drive motor to brake the shaft.

The Yale ERP20ATF park brake is released by the truck's logics when drive is requested by the accelerator pedal, and applied again when the accelerator is released plus the wheels stop rotating. Other signals, eg driver occupancy, direction lever, and key-on play a part in the logics too. It is a very effective aid for anti-roll back and ramp start, and is liked by drivers in my experience.