Changing the LHM
Changing the LHM and rinsing the system is a part of normal service and care for all hydraulic Citroëns. Don't neglect this if you want reliable and trouble-free operation from you car. Dirt in the system eventually wears out some of the components if it's not dealt with in time. Although the system is extremely robust in itself, this does not mean that you should tempt fate. If the flushing does not cure an eventual problem, then something has worn out and is not sealing properly, so a lot of the flow of LHM goes from the high pressure parts straight back into the returns, which leaves less for its intended purpose.
Replacing the LHM (either with fresh LHM or Hydraurinçage, the hydraulic flushing fluid) is slightly more complicated than on previous Citroëns, due to the various spheres and valves.
First, you have to depressurize the system. Put the suspension in the lowest position, on steering assisted LHD cars turn the steering wheel fully to the left (I suspect to the right on RHD, and I know RHD models are all fitted with DIRASS), this leaves the minimum amount of old LHM in the steering assist cylinder. After about two minutes with the engine running (do not open doors or tailgate or move the steering wheel during this time), loosen (do not remove!) the 12 mm bolt on the pressure regulator to empty the accumulator sphere, then turn off the engine.
It is of vital importance to observe the procedure to depressurize the system carefully. You can get the H I system stuck in hard mode if you, by error, remove the main accumulator pressure while the middle sphere is closed. When you first pressurize the car again and the system switches to hard for any reason (what can be triggered by simply opening a door), the car will move forcefully, potentially rupturing a sphere membrane. Whether it will jump or drop depends on the actual residual pressure of the middle sphere compared to the pressure in the struts. In either way, when the computer opens this sphere in the course of its self-diagnostics, the resulting jump or drop can be brutal enough to knock out supports or cause other serious damage. This is very dangerous.
Empty the LHM reservoir. The easiest way is to remove the refill cap, and to syphon out the fluid using a long tube. Then, remove the reservoir; this is done by removing one bolt, pulling out a safety pin, and taking off the uptake-return assembly holder (the big clip that holds the plastic with all the tubes coming to it). Have a completely air-tight nylon bag and some rags handy. Pull the whole container towards the front carefully, then tilt its front end upwards (it will be easier if you remove the uptake-return assembly first). As soon as you can, put the bag under and around the reservoir because there will be some LHM spilled.
Clean the reservoir thoroughly. Start with diesel fuel, then some gasoline, and finally detergent and water to get all the grime out. It is not very easy because the reservoir has a built-in labyrinth to prevent the LHM splashing around inside. The bottom of the container has special indentations designed to trap the crud (easily seen through the opening), be sure to clean them out thoroughly—using a hard bristle brush is a good idea.
By this time the uptake-return assembly will be drained into the bag, so you may dispose of it, but still keep the rags underneath. At this point you need to remove the uptake and return filters. The return is clearly visible, it's milky yelow-white plastic, about 4" deep and with a slightly oval cross-section. It is held by a small hook on one side, where it touches the rest of the assembly. The uptake filter is a long cone made of the same kind of plastic, hiding in the plastic extension of the uptake tube (the thickest one on the assembly) that goes into the LHM container. You can take it out by slightly twisting and pulling it out of its tube.
Clean the filters using clean gasoline. Dry out the container and the filters thoroughly with compressed air, then reassemble everything back to the reservoir, and the reservoir back into the car. Pour new fluid into the reservoir until the gauge rises to the top position, do not overfill!
You may need to prime the pump by removing the uptake hose and pouring some LHM into the pump, or even sucking on the hose to get the LHM flowing—use some kind of a vacuum pump, LHM tastes awful...
On cars with conventional steering, start the engine, then try to move the steering wheel. When you feel it is working as it should, the pump has started pumping. A few short races of the engine can cure the stubborn cases. If it does not co-operate, you will need to prime the pump again. Turn the steering wheel each way fully until the steering works without glitches, it usually takes one or two full turns from lock to lock. On cars with DIRAVI steering, I suspect that—unlike CX/SM—it does not need to be catered for specifically, because it is very low in the system so it's unlikely that air pockets would form. It should be enough not to move the wheel while the system is depressurised and without fluid (like in the middle of a fluid change), and then do a full turn left-right-left-right with the steering wheel once the pressure is back up.
Tighten the bolt on the pressure regulator. As you are doing this, you will hear the LHM filling the accumulator sphere, followed by the usual click of the regulator as it disengages. If you don't, you might have to do another short race of the engine, there might be an air pocket in the regulator or the pump. In any case, if the regulator clicked, the STOP lamp should have gone out.
Rise the suspension slowly. It is entirely possible that it will light the STOP lamp a few times as you do that as the air pockets clear out. Check the LHM level and replenish as needed, do not let it go below minimum. Keep rising the suspension until you get to the top position, then lower the suspension to the bottom position, and repeat the procedure. This time you should get no STOPs. When you have reached the top position again, you can lower the suspension to its normal operating position, or if you want, you can cycle it once again just to be sure.
At this point you should bleed the brake valve and the ABS block. These are the additional bleed points, but I have found they rarely need bleeding unless the system has been emptied of LHM by these particular parts being dissasembled. You can help matters by doing a small ABS test on a safe part of the road before bleeding out the brakes (careful, they might not react the way you are used to!). Bleeding the brakes will then take care of the remaining air bubbles. The brake valve is below the LHM container, its bleed screw is on the bottom side, it usually has a small green plastic washer on it—it is definitely difficult to get at. The ABS block is located on the left front wing, beneath the battery and behind the radiator. Not all types have a bleed screw.
You can bleed the front brakes without removing the wheels (but it will be simpler if you do remove them), at the rear you have remove the wheels to get to the bleed screw. Use a hexagonal socket that fits on the bleeding screw (don't try it with simple spanners. they would most certainly destroy the bleeder screw or even snap it off), a little can, a piece of transparent plastic tube about a foot long with 4 mm inner diameter. Also prepare something to put on the brake pad to keep it pressed down lightly, a piece of brick or similar. You might also need a small hammer, preferably brass, or a steel hammer and a piece of brass.
Put the car on level surface and suspension in the uppermost position, and the parking brake on just slightly, enough to keep the car from moving. Keep the engine running. Turn the steering wheel fully to one side to expose the bleed screw on the other side. There is a small rubber cap over the bleed screw. Loosen the screw with the socket until it just starts flowing. If the screw is stuck, don't force it, tap it gently with the small hammer. Attach the tube onto the top of the bleed screw, and put the other end into the can. Put the brick or whatever other weight you have onto the brake pad. Using a standard wrench of the same size, loosen the bleed screw and watch the fluid coming out through the hose. Keep it going until clear fluid with no discoloration and bubbles comes through. Then repeat the whole procedure for the wheel on the other side.
At the rear end, remove both wheels. Remembering to keep the car on park brake. Once you have taken off the wheels, return the suspension to the uppermost position. For the front wheels this only simplifies access, but for the rear one it is essential—the car uses the rear suspension pressure for braking on the rear wheels. The only way to have pressure in the rear suspension without the wheels supporting the weight of the car is to have the suspension in the top position. The procedure is similar to the front wheels: while the engine is running, loosen the bleed screw (the back ones are particularly prone to getting stuck, be careful), attach the hose, put weight on brake pad, bleed until clar fluid flows out of the bleeder. For the back brakes it usually takes a little longer than for the front ones.
Air bubbles in the rear brake lines are often responsible for strange braking effects on uneven surfaces, like sudden short failure of back brake to work after you hit a bump while braking (resulting in the rear jumping up). Also, the rear suspension comfort can be compromised by air in the lines because it makes the volume of the fluid in them compress—of course, it's the air compressing really.
The procedure is exactly the same for Hydraurinçage. The only differences are: it stays in the system for a shorter time, and it has a different color which helps a lot when bleeding. With the rinsing fluid in the system, it is advisable to clean the filters about each 500 km, so you get rid of all the crud the fluid dissolves. You need to remove the LHM container in order to get to the filters, the labyrinth in it will help with tilting it without spilling out the fluid, but be prepared for a mess (use nylon bag and lots of rags). However, it is well worth the effort—by the second or third 500 km period the filters should be almost clean, indicating that the system has indeed been rinsed out.