Hi Teun, I also have a Boley 5LZ.
Using the brake/clutch switch (C) to stop and then reverse the spindle at 4 times speed is the correct method for threading on the 5LZ. For short to medium length sections it's faster than disengaging the half-nut and using a thread dial.
I've had mine for a year and a half now and pretty much know my way around, although I'm not an expert.
I'm up for trading info and knowledge if you are interested. I have a complete English manual consisting of:
- Test Report BON 320/3E B1. 1
- Instruction BON 320/3 B1. 2
- Hints for fitting of Electric Multi-Disc Clutches
- Leaflet De 16
- Transport Instructions BON 715-6
- Erection Diagram BON 715 -7
- Controls and structural parts BON 715-8
- Headstock assembly drawing BON 715-9
- Leadscrew bearing BON 715-10
- Cutting Speeds in meters per min. BON 703 / 1
- Register of Change Gears BON 377/93
- Lubricating Instructions BON 705/1ae
- Instructions for d 32
- Wiring Diagram
Phil:)
Thanks all, 
Did some practice thread cutting with the half nut engaged and using the electric brake to forward and reverse the machine.......works great. Actually I do have a manual but it is in German 