Hallo cobminor,
the parking brake is moved by an spring brake.
The spring brake opens the parking brake with 3.5atü air pressure(3,5 kp/cm²)
the move way should be min. 46 mm to release the brake. You can measure it at the tie rod between spring brake and the brake cables. That means, the movement of the the cables of the rear axle sping brake at your ifa are to short. Give there more movement and try again. Oh, there is a little bit to much alc in my body.
edit without alc for better understanding. 
I had the problem, that my spring brake opens at 5,0 kp/cm². I changed the spring brake and after that the brake releases again at 3,5 kp/cm². There was mud in the spring brake, so it doesn' t work right.