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This system has given many people problems. Be careful with this system you need to have it looked at by a qualified mechanic soon. This is some useful info that might help. According to this you may have one of two parts failing and could cause the Auto Park to engage at ANY time even going down the road.
The Auto Apply Brake system on the Chevrolet P30 chassis is applied by a very strong spring that operates a cable back to the drive line mounted parking brake drum. Hydraulic pressure provided by an electric pump and fluid reservoir operates a piston that compresses the spring to release the brake.If the transmission selector is placed in park, and the ignition is in the run position, 12 volts is applied through a fuse to two switches, a low pressure switch which operates when hydraulic pressure is below 450 lbs, and the auto apply switch which operates when the transmission selector switch is removed from the park position. With the vehicle in park the pressure is below 450 lbs. The low pressure switch is operated and current is allowed to flow lighting the auto park lamp on the dash. With out hydraulic pressure the spring is at full extension pulling the cable to the parking brake at the back of the transmission fixed to the drive line. The emergency brake pedal is also attached to the cable. It is very easy to push on when the auto apply has been invoked.When the selector is moved from the park position to reverse or drive, the auto apply switch is operated allowing current to flow to a pressure valve solenoid, and the maintain pressure switch which is closed below 1200 lbs pressure and opens at 1600 lbs. The pressure solenoid allows pressure in the hydraulic system. At the time the selector is taken out of park the solenoid is activated holding pressure. Current flows through the maintain pressure switch and operates a relay which controls the high current flow to the hydraulic pump motor through it's contacts. Pressure begins to build in the hydraulic system pushing the piston against the spring, releasing the parking brake. When pressure reaches 450 lbs the low pressure switch closes removing the 12 volts provided to the lamp. However, there is another 12 volt path to the lamp from the maintain pressure switch which keeps the lamp lit. Once pressure reaches 1600 lbs, the maintain pressure switch opens releasing the pump relay, shutting down the pump, and shutting off the park lamp. If pressure should fall below 1200 lbs, the pump will start back up to maintain the necessary pressure to keep the brake released and the lamp will light as long as the pump is running. When the vehicle is put back into park or the ignition shut off, the auto apply switch is released, 12 volts is removed from the pressure valve solenoid dumping the hydraulic fluid back into the reservoir. As the pressure is released all of the switches return to their low pressure state.