It was obvious: M8 screws with 5 mm hexagon socket, where the tool cross-section is smaller than the cross-section of the screws. This could not end well.
Actually, I only wanted to disassemble the calipers, clean them and reassemble them with new screws to keep the look uniform on the machine.
After the 30 year old tool of my father was torn off at the third of four screws, I used a Dremel to cut a slot into the screw and the remaining toolbit. Unfortunately the screw still didn’t open, but I could turn the caliper and hold the screw with a screwdriver.
Torn off tool bit 
Tool bit broken inside the screw 
Cutting a slot into the screw head 
Drilled out screw heads 
Removed screws 
With the other brake caliper I drilled all four screws (the tool was broken). Surprisingly one could unscrew the screw remainders then without expenditure by hand.
Both brake calipers were then bolted together with screws made of stronger A4-80 steel. This corresponds to the original strength class. The higher screw head fits without difficulties and without collision.
| prev | 4581,48 € |
| Stainless Steel Screws A4-80 | 62,33 € |
| Sum | 4581,48 € |