Replacing Brake Pads - published in the BMWCC magazine

[Mike Fishwick]

BMW Brake Maintenance Part 2 – Replacing Brake Pads Mike Fishwick

(Orignally published in the BMWCar Club magazine)

Many owners regard the presence of a brake pad wear warning lamp as an excuse to ignore the condition of their pads, but the warning is only given when the friction material is worn to 3 mm, when there is a risk that damage to the discs will soon occur – particularly if the owner leaves replacement until the next service. A more serious risk is that as the friction material has now worn so thin, the heat-absorbing properties of the pad are reduced to virtually nothing. Heat will be then be passed through the steel backing plate and into the caliper piston, damaging the seals and possibly boiling contaminated fluid.

Removal of the pads is straightforward, the first step being to unplug the brake pad warning light connector on the inside of the wheel arch. Release the grommets for the pad sensor cable and the brake hose from their bracket on the suspension strut – these are usually very tight, but a little WD-40 will aid removal.

Now remove the long flat spring which holds the caliper in alignment with the pads. Simply insert a large screwdriver between the spring and the centre portion of the disc, levering to compress the spring until the small ‘ears’ at its inner ends are clear of the lipped holes in the caliper. The ends of the spring can then be pulled clear of the caliper bracket and the spring removed.

Lever out the caps covering the socket-headed pillars on which the calipers are mounted, and undo them – a 7 mm hexagon key which fits a socket set is invaluable for this task, particularly if they have never been removed before.

It is at this point – particularly on a well-used car with worn discs – that many owners experience a problem, in that they cannot remove the caliper. This is usually due to the outsides of the pads being trapped under the lip which has formed at the edge of the disc, and the inner pad being rigidly attached to the caliper piston by a spring clip. The answer is to use a large screwdriver to lever the pads apart, so moving the inner pad from underneath the lip.

The caliper can usually then be removed, complete with the inner pad, and sometimes the outer pad. In extreme cases of salt corrosion, the steel backing of the pads may be rusted to the caliper and its carrier, but careful levering should free it up.

You will now see the areas of the pads and caliper carrier upon which the pads slide, which should be cleaned with a sharp screwdriver or a small file until the pads are a comfortable sliding fit. In the case of new pads, check them before final installation, and carefully file them if necessary. Take care not to remove too much metal, or the pads will cause an incurable rattle!

Remember that if the pads cannot slide freely they will not be pushed back by the inevitable slight run-out of the disc and the compression of the piston seal, this problem being responsible for more wear of pads and discs than thousands of miles of normal operation. Needless to say, it will also slightly increase your fuel consumption.

Although many owners have a preference for ‘Genuine BMW’ pads, they are no better than other popular makes, and have the disadvantage of generating lots of brake dust, as do Pagid Fast road pads. Unless you are a regular track day enthusiast, avoid the temptation to buy ‘competition’ pads, as these seldom provide their potential without being thoroughly warmed up. Track fantasies apart, few of us use our brakes hard, and even then only for short periods, unless we often tow a heavy un-braked trailer.

The best advice on selecting pads is to look hard at your driving style, and buy accordingly – my preference is EBC Greenstuff (road-grade) pads, which work well in all conditions and do not generate much dust, leaving the wheels capable of being cleaned with a sponge and car shampoo. They wear well, too – those on my Z3 have so far covered no less than 50,000 miles, and have a lot of life left in them – but it depends on your driving style. For those with more extreme requirements, however, users of EBC Yellowstuff pads find that they are fine when cold, better when hot, and also have a low dust characteristic.

Before fitting new pads the caliper pistons must be retracted, to accommodate their increased thickness. While a special tool is available from Halfords at under £20, a large G-clamp and a small piece of plywood can be used, but take care that the clamp acts square to the piston. Slowly force the piston into the caliper, keeping an eye on the level of fluid in the reservoir, and sucking some out if it is near the top.

Contrary to popular myth, the warning light contact can be easily removed from the old pad and re-used in its replacement. With a pair of pliers, gently squeeze the ‘ears’ of the sensor clip onto the cable, and pull the sensor out of the pad in a radial direction. When replacing, note that the sensor will only fit the pad if in its correct position.

When reassembling your brakes do not be afraid to use an anti-seize grease such as Copa-Slip on threads and areas where the pads slide, but obviously do not use it to excess, as a thin coating will suffice. Make sure that the working areas of the discs and pads are clean and free from grease, the best way being to use a brake cleaner on a paper towel, and to keep your hands clean.

Extract the mounting pillars from the caliper, and lightly coat them with silicone grease to enable the caliper to slide freely on them.

Fit the inner pad to the caliper piston, place the outer pad in the caliper carrier, and slide the caliper/pad assembly over the disc and into position. The caliper will usually have to be tilted outwards at its base to align the mounting pillars with their threads. Now fit the flat spring, connect the pad sensor cable, insert the supporting grommets into their bracket on the suspension strut, and the job is complete.

With your brakes back together, the pads must be moved outwards against the discs. After a high mileage the master cylinders may have developed ridges at the ends of their usual travel, which may damage the piston seal lips if the pedal is pressed beyond this point. The pedal should therefore be slowly depressed in short movements until it becomes firm, this being a sign that the pads are now pressed against the discs.

On the road, test them progressively from low to higher speeds, and avoid hard braking if new pads or discs have been fitted. Although racers use heavy braking from new, their pads are of a far harder compound than ours. Light initial use of the brakes on a road car – for about 300 miles – will reward you in terms of longer life, better performance, and freedom from noise and vibration. It would be unusual for all the associated components to be so accurate that new pads align perfectly with the discs, so a period of bedding-in is obviously necessary.

When the brakes have been bedded in, or after a few weeks of normal commuting use, you may feel that your brakes have lost their initial ‘bite.’ Many owners have been assured that this is due to their pads having become glazed, and that replacement is the only answer! If well-maintained and sensibly used, expect a life of at least 30,000 miles from the front, and double that from the rear pads.

The reason for this loss of response lies with the manner in which our brakes operate, requiring that a thin layer of friction material is intimately bonded to the surface of the disc, a process which requires quite a bit of heat. During light braking this layer is worn way, and must be periodically replaced.

This of course means firm – but not heavy – braking effort, over gradually increasing periods until the brakes become really hot, with adequate time between for cooling. To brake from motorway speeds to 30 mph or so is obviously inadvisable, a long hill being preferable. You will then find that your brakes are wonderful again, and a decent level of braking should really be built into one’s weekly driving, not to mention before an MoT test.