No braking force

Thanks. We already did all that, no change

I would suspect the master cylinder (slow to bleed issues) or the load bias valve.
When replacing parts you must have total faith in the parts you're using. In the case of the load bias valve you could bypass it (temporarily) to be sure that's not at fault.
I wish you luck with this problem, I suspect the answer will be fairly simple when you find it. Chris

Yes, they have the load valves. We already changed that.
Today the Suzuki official workshop called to tell they have no clue whats wrong. So I dunno, really.
Flexi Pipes changed, valve changed, Abs pump tested, changed with a donor pump. Drums changed,main pump changed, calipers and cilinders checked, fluid changed... I will get a liter of gas and burn the car to ashes. ...

To me it keeps coming up master cylinder, don't forget dual circuit braking is estabIshed at that point. You tell tell us the front brakes are good so if the other side of the piston layout is faulty, there's your answer. A good handbrake (as it's not hydraulic) is barely relevant. You say "main pump changed" which I take to mean master cylinder. Changed to what? New part, reconditioned part or dodgy old second hand part? I say again you must be sure of any parts you fit or you will go around in a circle. Changing a master cylinder will never be a popular job as they can take a while to bleed properly. In the past I have back bled them using a motorcycle master cylinder and lever (that's pumping the fluid from the wheel end to the top) but of course requires a good selection of brake spares.
I can tell you are reaching desparation point as you feel you have tried everything.
You haven't, you've missed something. Good luck, Chris

So gents. Thank you all for your input and suggestions. Finally the y solved the riddle: there is a part that links the brake pedal to the master cylinder. This seems to have been badly adjusted so the pedal was not pushing with enough force on the cylinder, meaning not enough pressure from the cylinder, and only distributed to the front wheels by the bias valve. The workshop adjusted the pedal action, now the pedal is practically acting on the last 2 cm high end, and the braking effect is perfect on both axles.
Weird stuff, nobody really understands why the former owner made adjustments there. But in an end, I completely and unwillingly renewed the whole brake system.

It's almost always something simple that gets missed, we've all done it. The parts you changed were probably all past it so think of it as a wise investment. Chris