1979 Rivi Rear Disc Parking Brake Question

Hi from Germany,

I put new shoes on my rear disc brakes and adjusted the parking brake calipers correctly like discribed in this and other videos:

https://www.youtube.com/watch?v=k9FVCMFTXpA

I also adjusted the brake cable. With both wheels off, the parking brake engages after two clicks of the pedal, after 4-5 clicks I can't turn the discs any more.
Nevertheless, with both wheels, engine on and gear in "D" the car tends to roll unless I press the parking brake pedal down to the floor.

My question is if this is normal. To my mind a parking brake which acts with discs is never as effective as a drum parking brake, especially in our old cars.

Cheers
Michael