boney

You know parking brake pedal is held by friction of a spring around a cylinder. There is a toothed wheel on top of the cylinder that hold the wheel while connecting to the brake pedal. So as you apply and release the spring winds against the tooth wheel thus locking the force, so either the spring is weak or the toothed wheel does have enough teeth to hold on. Get it check at the service station, this is something we can only guess and do nothing.