The usual cause of this symptom is improper rear brake adjustment. Many vehicles utilize a self adjusting rear brake system. This system maintains adequate brake tension on the rear brakes so that when the parking brake is engaged, the rear brakes hold the vehicle in place.
The self adjusters re-tension the rear brakes each time the parking brake is engaged or the vehicle is driven in reverse and the brakes are applied. This system needs to be used frequently to maintain the proper rear brake tension. Often, the parking brake is used infrequently, especially on vehicles equipped with an automatic transmission. Due to this, the rear brakes wear and will result in excessive distance between the rear friction material and the rotor or drum. When the parking brake is applied, this excessive distance prevents the friction material from completely and tightly contacting the rotor or drum. This will prevent the parking brake from holding the vehicle.
You should apply and release the parking brake several times to activate the self adjusters. This may take as many as 10 to 15 times to bring the rear brakes into adjustment. If this is not effective, apply the brakes abruptly while traveling in reverse at low speed. If the parking brake still will not hold, the rear brakes will have to be inspected and adjusted manually. Once the brakes are in proper adjustment, you should utilize the parking brake on a frequent basis to ensure they remain properly adjusted.
PRECAUTIONS, TIPS, AND NOTES
Brake rotors and drums become extremely hot after vehicle operation and braking. Use caution when working on the braking system.