BRAKING SYSTEM MAINTENANCE

The pads and discs that are removed are an important source of information. For instance, the state of the disc must be observed carefully (state of braking surfaces, colour, profile). This examination can highlight faults in the functioning of one or more components (calipers, pads, bearings, etc.). It is important to resolve such problems before replacing the disc.

When do brake discs need to be replaced?
- When, during the course of a normal check, it is found that the disc's thickness is less than or has reached the minimum thickness indicated on the outer rim of the disc itself (MINimum THickness).
- When, while checking or replacing pads, cracks longer than 30 mm are found.
- When circular score marks are observed, deeper than 0.3-0.4 mm.
- When dark patches are found on the disc's surface.
- When, after a check, measurements reveal distortion or noticeable variations in height between a number of points over the disc's braking surfaces.

Lastly, before moving on to practical issues, we should bear in mind the following general rules:

- Instructions for the replacement of components should be read and scrupulously followed.
- Both discs on the same axle must be replaced on the same occasion.
- Make sure that the disc reference number corresponds to the vehicle on which it is to be installed. The same goes for the pads.
- Install two discs from the same pack (from the same production batch).
- Pads must always be replaced when discs are replaced.

REPLACING A DISC

DISMANTLING AND REFITTING

1) Remove the wheel.

2) Remove the pads and push back the pistons using the proper tool for this operation.

3) Dismantle the complete caliper from its support, without disconnecting the brake fluid ducts. Do not leave the caliper hanging from the flexible ducts. Suspend the caliper in some way (for instance, on a hook).

4a) Dismantle only the used disc, if this has a mounting flange inside the carrier.

4b) If the mounting flange is external to the carrier then it will be necessary to first dismantle the hub and then the worn disc.

5) Clean the new disc with an appropriate solvent (for example, petrol or spirit). The anti-corrosion layer must be completely removed. The disc must not be contaminated by oil or grease as these substances could be passed on to the pads and so reduce their performance. Carefully clean the surface of the disc that will come into contact with the hub.

6) Carefully clean the surface of the wheel hub on which the disc will be installed. Eliminate rust and other deposits. Check that the support surface is neither distorted nor damaged.

7) Check that play in the bearings does not exceed the tolerance and that the ball bearings rotate freely within their crown. If possible, adjust the bearing.

8) Fit the brake disc on the hub.

9) For discs with a mounting surface that is external to the carrier, fit the wheel hub and adjust the wheel bearing.

10) Once the disc has been fitted, use a DTI Gauge (fixed to the suspension bracket) to measure disc run out on a braking surface at the point of its external diameter. At the end of one complete rotation, run out should not exceed 0.10 mm. If it is greater, change the position of the disc on the hub (provided that the mounting aperture allows this), or fit the second disc from the pack. If the disc is held in place by one bolt only, secure it to the wheel hub by two other bolts (using wheel studs plus washers to compensate for the thickness of the wheel) in order to simulate rotation conditions once the wheel has been fitted.

Measurement of disc run out is a vitally important operation: if there should be too much run out then in time, after a few thousand kilometres, the disc may be subject to abnormal wear when travelling - and outside of occasions when the brakes are applied (due to slight rubbing against the pads) - and braking vibrations may occur.

The recommended maximum tolerance for disc run out has been established on the basis of our experience. Other sources may indicate slightly different values - in such cases the original value should be observed.

11) When the maximum recommended run out for the disc is exceeded then check run out for the bare hub. It should be borne in mind that the value recorded for the hub will double when measured at the outer diameter of the disc.

12) Fit the caliper to its support. A floating caliper must move smoothly and regularly along its guides. The pistons must move freely. Dust shields must be integral.

13) Fit the new pads - they must be free within their seats. Fit the springs and any other parts included in the kit.

14) Before fitting the wheel make sure that the rims are not distorted.

Balance the wheel/tyre unit correctly. Tighten bolts in the correct sequence and observe the recommended torque.

15) Check that the elements comprising the suspension are integral.

Check that the shock absorbers function properly. The suspension adjustment must correspond to the manufacturer's recommended values.

TESTING AND RUNNING IN

Once the discs and pads have been replaced, the mechanic should then carry out a road test. He must ascertain that there are no brake vibrations or noises, either while travelling or during braking. He must also check that the braking action is both correct and efficient, even though the brakes are not yet run in. It is the braking distance that is important. Never brake sharply during these tests.

The car user should be advised to observe an approximately 200 km running-in period. During this period a short, smooth braking action should be adopted so that the pads can align correctly to the disc surface. Too sharp or heavy braking may not only cause the pads' friction material to overheat, but also the disc itself. This would end up by compromising brake integrity and performance. In particular, do not attempt to activate the ABS.