After having tested the water-cool system I noticed that after about 2 minutes the whole system was filled with bubbles caused by the water passing a T-Splitter at high velocity.
This T-Splitter was installed for easy maintenance and filling of the the water-cool system.
The 90deg angle in this T-Splitter was so sharp and the water-velocity so high, it created water-vapor bubbles.

The solution to this problem was to slow the water down at this T-Splitter without loosing water velocity in the rest of the system.
Thats why I designed, build and installed this T-Splitter-Diffusor successfully.

This diffusor slows water velocity down 8 times, after which it passes the T-Splitter and then increases water velocity 8 times again.
After having installed this diffusor, there were no more water-vapor bubbles.