Adjusting the parking brake

Remove the rear wheels.
Undo the handbrake lever and push back the disc brake pads of the rear axle until the brake disc can rotate freely.
Undo the adjustment nuts on the turnbuckle (arrow) until the cables are without pretension.
NOTE: To make the turnbuckle accessible, open the cover of the tray behind the parking brake lever and remove the rubber inlay and also the insert.

Fig 1: Adjusting Parking Brake Adjusting Nuts On Turnbuckle
Courtesy of PORSCHE OF NORTH AMERICA, INC.
With a screwdriver, adjust the adjustment fixture through a threaded wheel bolt hole until the wheel can no longer be turned. Next, turn back the adjustment fixture through 5 notches again (release approx. 3 notches until the wheel can be turned freely, then release another 2 notches). Repeat the procedure on the other wheel.

Fig 2: Turning Adjustment Fixture Through A Threaded Wheel Bolt Hole
Courtesy of PORSCHE OF NORTH AMERICA, INC.
Pull up the handbrake lever by 2 teeth and turn the adjustment nut of the turnbuckle until both wheels can be turned manually with difficulty.
Release the parking brake lever and check whether both wheels can be turned freely.