wrote:

use the "spring tension adjusting screw" (#2 in the diagram) to
vary the position of the arms in relation to the rim.
The spring itseelf is missing in the diagram I would say. I solved it by
bending the spring (the long iron "wire") outwards a bit. It was not
possible to completely get it right using the adjusting screw. I had to
bend the spring slightly.

Thank you all very much for the help: you gave me lots of useful tips.
I find it far more difficult to adjust then a normal Shimano caliper brake
for racing bikes.

Greets, Derk