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The one and only brake fluid is called LHM

The one and only brake fluid is called LHM

The critical point in the brake system is the brake fluid. Citroën does again a solo with the brake fluid. All today's cars use DOT 3, DOT 4, DOT 5… as brake fluid. Only Citroën uses LHM, a green hydraulic oil.
But be careful! The early 2CV's need DOT the later models LHM. If your 2CV has front drum brakes than it needs DOT. If you find disc brakes than LHM is required.
To tell LHM components apart from DOT components all LHM parts have a green mark. Master- and wheel brake cylinders are painted green or have at least a green dot. Even the small seals for the brake lines have a green dot. The hydraulic oil LHM is green as well. When the car was new, there was a sticker on the brake fluid reservoir. This sticker told in multiple languages that only LHM is good for this brake system.
Similar information is on the cap of the reservoir. Mostly the sticker has gone and the cap is covered with dirt. But even if everything was clean nobody in a workshop will read this! Not because the mechanic can't read, nobody ever has the suspicion that a 2CV needs a different brake fluid than every other car.

So they use DOT in a disc brake system and destroy the entire system. Both fluids are very aggressive against the rubber seals of the other system. If you use DOT in a LHM system all rubber seals will disintegrate. So the complete brake system will fail sooner or later! Of course the very same will happen with LHM in a DOT system.
Experience shows only a view mechanics know about this fact. Only a little amount of DOT is enough. If somebody tops up with a little sip of DOT, the LHM brake system is "mortally ill"! The first problem will occur with the rear brakes. The rubber seals in the wheel brake cylinder start to disintegrate and seize up the pistons.
If you don't hit the brakes very hard the rear brakes don't work at all. This enables the brake shoes to seize because of the rust. On the front brakes you will find out about this problem when you try to replace the brake pads. The pistons can't be pushed back into the calipers.

Finally the master brake cylinder seizes. This means you push the pedal like usual, but when you release the pedal the brake pedal won't come back out. It takes a few seconds before you have pressure again.
If you now open the cap of the reservoir you might find a black slime on the bottom of the reservoir and on the screen. This slime is the disintegrated rubber. A change of brake fluid is already too late. The seals are destroyed.
If you catch the replaced brake fluid and let it settle for a moment, you can clearly see the two fluids separated. In the middle between the fluids the black stuff is rubber particles. If this is the case the entire brake system has to be replaced. If you try to flush the system with fresh LHM you will find that there is still some old DOT fluid in the calipers and the rear brake cylinders. This is enough to start the destruction all over again. You should be aware of these problems. If you give your car to a mechanic educate him know about this fact. Tell him and make sure that he understands. Your life may depend on this!