GRRR, I wanted to post results now, but first i needed to close some holes in the block. I used such a saw for the for the channels between the jets so these channels are on the whole block, not only in the center:

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To close them i wanted to solder a litte copper plate over the holes.
Everything worked well until now but here the trouble begins:
It took me nearly 4 hours (from 6 to 10 o'clock), but i couldn't attach these fu***** copper plates. Therefore i have 2 other leacking areas now.
I have soldered all my other copperblocks i have build yet and none ever made any trouble, but this one is that damn hard to solder because of its huge mass.
It took three beers and four filled camemberts to calm down but now I'm ready to start again. This block won't beat me i have already solved greater problems, this block just can't beat me!