With the car on a level surface, with the handbrake on and jacked up and a suitable axle stand or wood block placed under the car in case the jack fails, remove the wheel. (never work on a car supported by a jack alone)
[shadowbox=http://c4owners.org/images/c4/large/brake2.jpg|Brake Assembly||right][/shadowbox] With the wheel removed you will see the brake disk and caliper assembly:

The caliper has 1 side fixed and the other side sliding. Brake pressure is applied via a large piston in the sliding half operated by hydraulic pressure from the foot-brake.

The inner pad is pushed against the revolving disk first and when it can travel no further the other side of the caliper is pulled in applying pressure to both sides of the disk. (see the image above)

This means that due to varying braking pressure the inner pads will wear down first and these are the hardest to see.

[shadowbox=http://c4owners.org/images/c4/large/brake1.jpg|Tailgate Lock||left][/shadowbox]
What you are looking to replace are the 2 brake pads and 4 spring retaining clips on each brake assembly

To remove the pads and retaining clips you first have to split the caliper by removing the retaining bolts but first using your screwdriver between the inner pad and the piston gently ease the piston back as you will need the additional room as the new pads will be thicker than those removed.

WARNING: Before splitting the caliper see if the sliding pad moves freely and that there is no sign of any brake fluid leaking. If the caliper won't move or there is signs of fluid then put the wheel back and seek specialist advice form your Citroën Dealer

The first time you change the pads the disks should be OK. However if there is any signs of 'crazing' on the disk surface or the wear looks excessive you may need to replace the disks themselves (at 43,000 mine had worn down by circa 0.5mm - 1mm per side and may need replacing next time the pads are)
[shadowbox=http://c4owners.org/images/c4/large/brake5.jpg|Brake Assembly||right][/shadowbox]
Important: The minimum thickness for the disks is 20 millimetres which has to be checked using a micrometer or similar tool. The C4 also has a [shadowbox=../../images/c4/service/wear.jpg|C4 Brake Disk Wear Indicators||]disk wear indicator[/shadowbox] via 2 slots milled into the outside edge of the disk which show how much wear there has been. Although the minimum thickness is 20mm the grooves are a better indication of actual wear. If the edge looks 'Sharp' it's time to change the disks.

Assuming everything looks OK split the caliper by removing the 2 retaining bolts (see first image for location) using a 13 mm spanner. The bolts are standard right hand thread with no locking devices.

Assuming everything is still OK remove the old pads and spring clips by simply levering it out with the screwdriver. Clean the assemblies down with a soft brush or a fine wire suede brush being careful not to breathe any of the dust in and keeping everything clean and free of grease or any other contaminants - then fit the new ones

[shadowbox=http://c4owners.org/images/c4/service/brake_schematic.jpg|C4 front Brake Schematics||right][/shadowbox] If the disks are to be changed then you will require a T55h and T30h Torq spanner. Basically these are 'Star' shaped socket wrenches which are similar to the hexagonal ones you get with flat pack
furniture. However they are designed to be used with power tools for fast assembly on the production line.

As the caliper is already split all you have to do is remove the 2 bolts holding the fixed caliper section. They are at the back and are tight! - You may need to use a lump hammer to get them to move initially. Once removed, take the caliper section off and remove the 2 countersunk screws holding the disk in place with a T30 spanner and remove the disk.

Once it's off give the boss on the end of the drive shaft a quick clean and also use some thinners to clean any protective coating off the new disks and then fit the new disk and the 2 retaining screws. Then refit the fixed caliper ensuring the bolts are TIGHT!

After that it's a case of refitting the sliding caliper and new pads. Don't re use the old pads with new disks. Fit new ones as your life depends on the brakes on your car.

[shadowbox=http://c4owners.org/images/c4/large/brake4.jpg|Brake Assembly||right][/shadowbox] Before you re-assemble the caliper make sure the piston is back flush with the housing so it can clear the new pads when you try and put it back on. It may require some (but not excessive) pressure to press it back flush.

Next fit the new spring clips first with the side with the central 'Lug' pointing out from the disk

Then slide the pads into place.

Then re-fit the caliper and make sure the bolts are tight.

Do not apply any grease or other 'release agents' as they can lead to failure of the brakes! You could apply a 'sparing' coat of Copper slip or a similar High Pressure/Temp Copper Based Grease to the piston end to reduce potential 'squeal' but be very careful!!!