The parking brake should hold the ar with between 7 and 8 clicks of the hand brake lever. Jack up both rear wheels and get a friend to slowly apply the hand brake as you rotate each wheel, the hand brake should apply evenly at each wheel or one of the cables may be seizing.
To adjust the brake tighter the adjusting nut is found at the hand brake lever its self. Fully pull up the lever, on the under side of the lever is a 10mm or 12mm nut turning it clock wise will tighten the handbrake, ensure after adjustment the brakes do not drag.