Brakes, wheels and tires

Brake clonks while reversing

This is common and is not a fault only a nuisance--it is because of the way the calipers sit. When in reverse, and you brake, the force on the assembly is in the opposite direction as when you drive forwards, and the small amount of play in the calipers make them clonk the other way. This also happens on Peugeots which also have the same assembly. Original brake pads behave better than cheap pattern ones.

Martin GUTKOWSKI

Rising back end while braking

This is a sure sign of air in the rear brake circuit. Bleed the whole system including the brake valve.

Chris STUART

Selection of tires

A tough question. It depends on your requirements, driving habits and, last but not least, the amount of money you are ready to spend. Many of us find Michelin tires to outperform other brands mainly because their different, softer composition is better suited to the suspension and riding characteristics of Citroëns. I also found them to last much longer than, for instance, the Pirelli tires of the same price and performance class I had on the car before. But all that having been said, you will find other BXers who left Michelin for another make and don't regret their choice.

My personal opinion is that unless you want to exploit some special sporting characteristics of other tires or their different grip on dry or wet road surfaces (and you have to experience this for yourself, handling under different, sometimes extreme conditions is not something that could be described exactly by words), you should better stick to Michelin if you can afford it. And don't forget that cheap tires are generally more expensive in the long run as you have to buy a new set much more often. True, you'll have to pay in installments rather than all at once...

And then comes the evergreen debate of tire sizes. I share the opinion of Kjell T Svindland ("Changing to wider wheels for cosmetic reasons is generally a bad idea. Citroën engineers know what they do when they select a certain tire dimension for a certain car, and going up even as little as 10 mm on the width may deteriorate the handling and directional stability on bad roads."), Malcolm Gray-Stephens ("The trouble with wider tires is the unsprung weight; as the tires are heavier the ratio of the car on springs to wheel weight becomes worse; this gives poorer acceleration (inertia), poorer ride (but quicker response). And don't think that you can counteract the increased tire weight by using a light alloy wheel: most of them are not lighter but even havier than steel wheels. Wider tires can also disturb the steering weighting as the centre of the contact patch of the tire moves in or out relative to the pivot centre of the hub. It also results in increased wind resistance.") and Željko Nastasić ("Grip is proportional to drag which is proportional to pressure. On slippery surfaces, wider tires actually offer lower grip because the pressure is lower: the weight on the tire is distributed over a bigger area. Also, the tire profile and the height (tire outer diameter versus wheel diameter) influence comfort greatly. For instance, the difference between 195/15-60 and 195/15-65 on an XM is almost too big to believe: the slightly higher profile offers a much better ride comfort, but admittedly slightly worse handling in extremes (in tight curves, traversed at highly excessive speeds courtesy of hydractive suspension, you can actually hear the rear tires bending and producing a flop-flop-like noise as well as feel it in the movements of the tail of the vehicle").

However, others quote their different experience: Geir Korneliussen: "With a slight modification of the rear wheel arch wider tires can be fitted to the BX. On 14" I have tried 195/60 and it gave a nice smooth ride comfort. Today I use Enkei Sport Racing 7"×16" wheels with Michelin XGTV 215/45 VR 16 tires. This combination gives my car extremely precise handling and a roadhold my friends envy."

Cleaning tires

Avoid silicone based products, they are rather harmful, although it might seem different at first sight.

The long hydrocarbon chains that make up the rubber of the tires are attacked by both ultraviolet rays and ozone. As the molecules break up, the rubber loses its elasticity. The manufacturers use carbon black to protect the rubber against UV rays (this is why we only have black tires). A wax based protector is also used: as the tire rolls on the road, additional wax is forced to the surface and absorbes the harmful effects of ozone.

Silicone based products usually contain no UV-protector. They can also dissolve the wax protector and cause sidewall cracking and failure. Hence, although you can use them to refreshen rubber hoses and parts under the hood, avoid using such products on tires and rubber trim which are exposed to direct sunlight. To protect those, chose a product that contains UV-protectors and has no silicone.

Changing wheel bearings

Undo the driveshaft nut (get someone to stand on the brakes while you do this, jack up the side concerned with axle stands, remove the anti-roll bar top swivel). Wire brush the threads as the nut disappears at the end, remove the steering arm and the brake caliper, tie them up, remove the hand brake cable bracket and the brake disc, remove the lower swivel, the driveshaft nut, the bolt holding the hub to the strut--be careful not to pull the driveshaft out of the gearbox or else you will lose the gearbox oil. Now you have the hub in your hand.

Drive the inner hub out with a hammer and 3/4" socket or similar. Remove the 3" diameter circlip carefully with a screwdriver or an internal circlip pliers. You may need to break the rust out first by chasing it around with a punch and hammer.

Try and drive the bearing out—it is into a blind 'L' lipped hole so you cannot get behind the outer race and the bearing will break up when you hit the inner race but do it anyway (note that it is the dried grease that failed not the bearing). Now you are left with an outer race hard up against a blind lip. Get a 2 mm or thicker lump of metal that is small enough to fit the outer race in several places, MIG weld this lump/plate to the bearings at several places, hit it hard, it will break away from the hardened outer bearing race, reweld, hit again. After about 7 cycles enough of the outer race will have come away that it cracks, now it will hammer out of place no problems. Basically you are using the heat from the MIG to loosen it and the weld to get something to hit.

Clean up the housing, grease it, and tap the new bearing in hitting the outer race only, using a piece of wood as a buffer. When the bearing gets flush with the mounting, use a socket and tap the edge of the bearing going through 360 degrees. Finally, refit the circlip.

Malcolm GRAY-STEPHENS

You can remove the wheel bearing assembly with a vice, hammer and chisel. All you have to do is to remove the inner race by removing the circuit. The inner race is in two halves that can be easily tapped out (it will even fall out if it is already badly worn).

Then comes the difficult part. The inner race is pressed and will only come out in one direction, backwards to the gearbox and driveshaft. In the centre of the outer race there is a small raised section. Place the chisel or screwdriver there and hit it, then move the tool by 30 degrees and hit it again. You need to keep going round the full circle, the outer race will move quite slowly. Once the edge and face become visible, it is easier to hit the edge rather than the centre of the race. Be careful not to damage the face of the bearing mounting.

Neil PARRY

Intermittent ABS failure

The most likely cause is intermittent connection to a sensor (or possibly an intermittently failing sensor) which is OK with the car level and the wheels in the straight ahead position, but which breaks when wheels are turned or the body tilts.

Measure the impedance of each ABS sensor coil on each wheel with all possible combinations of suspension at maximum and minimum level and steering turned fully to the left and right. If that doesn't work, you could try detaching each sensor from the wheel and gently flexing the cable.

If the cold resistance seems to be OK, attach a sound amplifier of some sort (a walkman or similar) to one of the sensors. As you drive the car, you should hear a beep whose pitch is proportional to the speed of the car. Listen for any sudden change or dropout as you turn to the left and right. Check the remaining three sensors as well. If the sensors seem to be all right, repeat the previous test with the amplifier attached to the sensor inputs of the ABS computer.

Although a failed sensor is probably the most common ABS fault, this is not the only reason the ABS light can come on. Failure (or intermittent operation) of any of the wiring to or from the ABS control unit, failure of the ABS relay (located on the right side of the steering column support bracket, accessible once you remove the lower shroud) the fuse #14 in the main board, or failure of the hydraulic control block will also put the light on. It is also possible that you have a sensor or wiring which works fine when cold, but becomes open circuit when it warms up. The sensors themselves shouldn't get particularly warm, but the wiring might.

Martin WISEMAN

Brake valve

Although there is a so-called repair kit for the brake valve, this only contains replacement rubber seals. These seal the valve to the outside but they have nothing to do with the functioning of the valve itself; the sealing of the working elements rely on the very close tolerance between those parts. Once the pistons inside the valve or the cylinder they travel in wear out, the valve starts leaking (this means leakage into its return lines and not to the outside world).

Before you can judge how much the valve is worn, you have to clear it of debris and have the LHM in relatively good condition. If you haven't done it recently, flush the system of the BX or of the XM. In some cases it is the debris itself which obstructs the proper operation of the pistons and their sealing. Only when this is eliminated as a potential problem should you diagnose the valve itself.

There is a test you can do: let the car roll (contact on) after engine is stopped at walking pace, then step on the brake hard. Keep pressing. If the back continues to drop, or, even worse, the STOP light comes on within a couple of seconds, the brake valve is worn out. What happens is that the leak in the valve will continually drain the rear suspension, which will then attempt to keep its height by replenishing from the accumulator. Without leaks, this is soon accomplished, with leaks, the accumulator pressure basically just escapes via the brake valve and the suspension continues to drop, eventually the STOP light will go on as the pressure drops below the safety valve pressure threshold.

To accomplish this you have to uncouple the return lines; there are two of them routed independently to a common return collector right behind the LHM reservoir. This collector also collects leakage returns from three other elements (this depends on model and engine, as well as the particulars of the hydraulic system as there are several variants). The only reliable way to test the valve for leaks is to uncouple the return lines at the valve; be prepared that this is rather difficult to do beacuse of the poor access. You need to decouple both return lines and plug the hoses so that the other returned LHM does not leak out through them. On some variants the return hoses are retained on the valve by an additional clamp holding them in place and this makes it even more difficult to remove them.

When all done, observe what is coming out of the valve return lines themselves. Be careful not to press on the brake pedal while the return lines are off or else you will spill a seizable amount of LHM as pressure is released from the brake cylinders. The pressure involved is rather high so you'll end up making a mess.

If the valve does leak profusely (what is the prevalent reason for the rear end sinking), the only solution is to replace it. The valves are more or less similar accross the Citroën model range: BX, CX or even late DS models; there are, however, subtle differences. For one, the XM valve is slightly modified to reduce shortcircuiting the rear suspension into the return when the brake is released, plus the pipes are connected slightly differently (it is possible to connect a BX/CX valve in the same fashion).

But if you want to open it up anyway, here it goes: pry out the ring holding the rubber boot in the front (where the brake pedal was located), then remove the boot and the seal behind it. Remove the retaining ring at the opposite end of the brake unit as well. Carefully tap on the plug behind it until it starts moving outwards (clean the area thoroughly before you start). Be careful, there is a very strong spring behind this plug. If it doesn't want to come out, try pressing the main piston all the way inside (from the other end where you removed the rubber boot from). Don't apply too much force, however, as without pressurized LHM the pistons get no lubrication and can be damaged.

Now remove this strong spring and the cap which is behind it. Press the main piston all the way in again to get the two-piston assembly out in the other end. Take care not to damage its surface and the teflon seal it slides in. If this assembly slides out very easily, you might have to start thinking about a new valve, as this is a large contributor to leakage and the sinking rear end syndrome.

Don't disassemble this two-piston assembly, you cannot and you need not. But be carefuly with the little spring coming out after having removed the assembly. Now you can remove the retaining ring on the front side, which lets the main piston and another small spring behind it come out. Note which small spring came out from which side, they must not be mixed up. Use a long and blunt instrument to push out the remaining middle piston through the back end.

Remove the bleed bolt (if its tip is rusty, it is wise to think about an overhaul of the rear calipers and a good blow-through of the pipes going to the rear brakes). Clean everything thoroughly. If the pistons have large or very uneven patches of shiny, mirror-like wear, you may need a new valve soon. You receive a new O-ring for the back end plug in the repair kit but the teflon ring the two-piston assembly slides in is not replaceable. Douse everything with fresh LHM before reassembly. Be very careful of the orientation of the middle piston; if you put it in the wrong way, the rear brakes will block when brakes are first applied, and will not let go until rear suspension pressure is released. Reassembling, as usual, is in the opposite order, the last step is to get the back end plug back in its place—and with the spring behind it, you'll have lots of fun with it...

Željko NASTASIĆ