Should we put some brake fluid in fuel tank and how much. Is it true that this will lengthen the life of fuel pump and injector?
Urban myth. It will do no good at all IMHO. If you are using Ultra-Low Sulphur diesel (ULSD) that typically is about 15ppm of sulphur you may be well advised to use a specialist fuel treatment to improve the lubricating qualities of the fuel. Sulphur is a very good lubricant and taking it away is good in one way (improves emissions) but not in another (lose lubrication to the fuel pump). Your fuel supplier should be able to advise you on suitable fuel treatments.
I use power service and it works good, i've heard of using ATF every once and a while but i've never heard of brake fluid.
Ya break fluid would be bad, I cant think of anything that would be beneficial. To add on what Nige said about low sulfur diesel....Low sulfur has been around for a long time, if you have been using on road diesel most seal damage would have already been done and you would have seen problems before now. Adding power service, atf, 2stroke, even clean used oil can help with lubricity but you dont need much.
Most brake fluids are alcohol based, I'm not sure what the benefits are supposed to be in a diesel. I've never heard of adding it to fuel, but as eric12 mentioned, ATF has the same lubricants that old style diesel fuel had in it before they brought in Ultra-Low Sulfur grades.
Brake fluid in fuel is a absolute NO NO. It will swell many types of rubber. ATF is not so hot for a additive either. The new additive package in ATF is a friction modifier. More grabbing action to clutches. more friction for precision parts. If you want a great lube additive for diesel, add 2% soy diesel or WVO to the fuel. The cost is nothing as just replaces dino diesel at about the same price. Another additve used frequentlly in Cummins truck engines is TW3 2 cycle oil. The cheap stuff from Walmart is just fine. The normal mix is one ounch per gallon of fuel. No synthetic please. It is hard to beat fresh CLEAN USLD in the tank and loading the engine for several hours. My statement on trucks is hang a nice heavy trailer on back and put her in the wind for a few hous. They will run better and quiter after this workout.
Brake fluid is corrosive. As stated above, there are several additives that are better suited. 2 stroke oil is probably the best value for the price. Jey C, which part of France are you in?