Just a follow up if someone face the same problem : Despite all efforts, I was not able to stop the leak So, I finally decide to replace the steering fluid cooler. I didn't mount an OEM, I bought a Hayden kit from ebay and mount the cooler in front of the engine water cooler :
I plug it directly to input / output of the steering system (it bypass the fluid cooler tube, the red part on the schema above)
I mount it a week ago, and since, there is no more leak at all ! (and I did city driving plus 500kms of gravel and offroad, so, it has been tested seriously ) thanks again Rebelord for you advises =]