Parking brake on, car somehow shifts a bit when the engine & power turned off

[AxelDoobie]

I noticed something a bit weird today and I'm not sure if its normal. Usually when I park on an incline I put on the parking brake before shifting into park so the car stays in a solid position without shifting or applying pressure to the parking pawl. The weird part is that after I turn off the power by pressing the engine button twice (first to turn off the engine, 2nd to turn off the idrive/music/lights/etc), the car then rolls about 1/2 an inch towards the decline. Any idea how this is even possible? The parking brake was fully engaged and that should prevent the car from moving at all. Also this 1/2 of slack movement doesn't happen until I turn off the electronics (my foot was already off the brake beforehand).

To reiterate...
1. I pull into my parking space & stop completely (foot on brake)
2. I turn on the parking brake and feel the pedal sink in & see the red light turn on
3. I shift into park & release the foot brake (car continues to stay still)
4. Press power button once to turn off the engine (no movement)
5. Press the button again to turn off the power (then car rolls/shifts a bit)

Assuming this is not normal, what the heck is going on?

[CatalinP]

It has to do with how the parking brake works.

When the engine is on, the Auto Hold / Parking Brake works on all 4 wheels and it is controlled by the ESP/ABS module which puts and maintains pressure in the brake lines. Once the engine turns off (or even after the engine is idle for several minutes) the parking brake is shifted to only the classic cable driven parking brake on the rear wheels.

Nothing to worry about.

[AxelDoobie]

Originally Posted by CatalinP

It has to do with how the parking brake works.

When the engine is on, the Auto Hold / Parking Brake works on all 4 wheels and it is controlled by the ESP/ABS module which puts and maintains pressure in the brake lines. Once the engine turns off (or even after the engine is idle for several minutes) the parking brake is shifted to only the classic cable driven parking brake on the rear wheels.

Nothing to worry about.

Interesting... although I do feel its a bit unnecessary for the parking brake to alternate between two separate systems like that. I guess this means there is no proper way to park on an incline without putting stress on the parking pawl

[CatalinP]

The parking pawl is designed to take "some stress".

[suitedcboy]

I do not think the shift you feel is onto the pawl, it is the park brakes shoes shifting to the anchor block. When I park with park brake then release is not with thump and a driveshaft wrap as it is with no park brake. The drum brakes get pushed out against drum by the expander but that has a set of shoes lock against drum and expanded away from from the anchor block on the backing plate. When the wheel brakes (the auto hold function being deployed) releases the park brake shoes have to move and stop on the anchor block. If you adjust the park brake then the motion is less as the expansion is less and thus distance to the anchor block when the park brake takes over. The holding force is high for these systems but if it is already rolling it would need to be higher so having it held when applied fixes the issue. Many/Most electric park brakes require brakes depressed in order to apply. I have foot brake operated drum in hat park brakes on other vehicles and the same give is felt on many of those. The ones with star wheel adjusters are better when adjusted. I could lock park brake with the vehicle on my lift and have a few degrees rotation either way if I moved back tires. Adjusting park brake shoes reduces the travel and the motion goes down. If this happened suddenly then one of the linings may have come off the shoe and that should be eliminated as cause.