Brake bleeding is one of the most cost-effective maintenance tasks you can perform, an annual operation removing most of the impurities which accumulate in the lowest parts of the system. These include moisture, rubber particles worn off the piston seals, and metallic debris from cylinder bores. Moisture is absorbed through the rubber hoses and into the hygroscopic brake fluid, reducing its boiling point, and as the “wet” fluid is denser than when “dry” it will gradually find its way down and into the callipers, which are the hottest part of the system.

It is possible for any moisture in the caliper to boil and generate steam, which being compressible will permit excessive movement of the pedal, and a lack of braking, with a similar feel to that of brake fade.

In spite of the propaganda that the braking system of a car with ABS – or ASC/DSC – can only be bled by a BMW dealer, there are no problems for the DIY maintainer, at least so long as there is fluid in the system. As the ABS system is dormant in such conditions, fluid can be forced through the system to any brake which is being bled.

I have used a Gunsons pressure bleeder for over twenty years, which has pushed many litres of fluid through my Z3′s braking system, and even after removing the brake calipers I have never experienced any problems. As a bonus, a pressure bleeder does not require the pedal to be depressed, and also enables the task to be performed easily by a single person.

An obvious, but little-appreciated consideration is that with any dual-circuit brake system the use of a pressure bleeder is essential, for if bled by normal means, the first circuit to be completely free of air will then go “hard” and prevent the pedal being pressed sufficiently far to fully bleed the other circuit. The pedal may feel firm, but it is only operating fully on one circuit – hence the problems which many people experience at MoT time, when the required level of brake balance cannot be achieved.

Needless to say, before embarking on bleeding your brakes, make sure that you have everything to hand, including small ring spanners which fit the bleed nipples. In the case of my Z3 this means a 9mm for the front calipers, and 7 mm at the rear. If you intend to purge the contents of the caliper cylinders, a piston retraction tool will be required, which can be purchased from Halfords for under £20. Alternatively, a suitable g-clamp and a piece of plywood will suffice.

If the bleed nipple hexagons have been damaged, a set of spare nipples will be useful, as will spare dust caps and some stiff wire. The wire will be useful to clear a blocked nipple, which can easily be the case if a dust cap has not been replaced.

Your fluid will doubtless be of either DoT 4 or DoT 5 specification, DoT 5 having a higher “Wet” boiling point, but DoT 4 a higher “Dry” figure. If you are going to bleed and replace the fluid regularly, DoT 4 will be quite adequate. I usually buy ATE DoT 4 fluid by the litre at German & Swedish outlets.

In these days of rapid servicing many “professionals” carry out a fluid replacement by simply sucking the fluid out of the reservoir and then topping it up, which may perhaps be considered as replacement, but to quote Star Trek – “Not as we know it!”

I usually replace my fluid every two years, first using the empty pressure bleeder to bleed the right-hand front brake until the fluid is at the base of the reservoir, then bleeding another litre of fluid through the system.

Simply put half a litre of brake fluid into the pressure bleeder, fit it to the brake reservoir, and connect to a suitable pressure source – I use an old spare wheel, inflated to about 25 psi. Make sure there are no leaks, and then simply work around the car, bleeding each brake until clean fluid has been issuing for ten seconds of so. Close the nipple, giving the spanner a firm pull with two fingers, replace the dust cap, and continue, keeping an eye on the level of fluid in the pressure bleeder.

Although the approved order of attacking the bleed nipples is right then left at the rear, followed by right and left at the front, the important thing is to bleed the braking circuit at one end of the car before moving to that at the other end.

Remember to also bleed the clutch slave cylinder, as this is subjected to much more mechanical movement than the brakes, and so tends to wear more. It will bleed much faster than the brakes, due to its larger-bore pipework, so take care not to exhaust its fluid supply, which is the upper portion of the reservoir contents. To fully bleed the clutch slave cylinder from “empty” will require its removal and inversion to position the nipple at the highest point.

The only problem you are likely to encounter is that of knocking over the jam jar which most people will use to collect the old fluid. I prefer to use a large screw-top tin with the bleed hose passing through a hole drilled through its lid, which is more stable and less likely to dump all of its contents over the garage floor. A large ice cream container is a suitable alternative. Whatever type of closed container you use, make sure it has a breather hole so that the pressure bleeder does not pressurise it, making removal of the bleed hose a messy experience!

Every few years, or whenever the calipers are removed to fit new pads, invert each caliper so that the opened bleed nipple is at its lowest point, and with the pressure bleeder connected use a suitable tool to slowly press the piston into the caliper to expel the contents of the slave cylinder. Remember that the caliper cylinders are not purged of their contents when they are bled in the normal manner, and being the lowest point of the hydraulic circuit, they act as collecting points for all the moisture and dirt in the system.

Should you wish to change the fluid, use the empty pressure bleeder to bleed the fluid reservoir to its minimum level, and bleed half a litre of fluid through the rear brakes, followed by the same treatment for the front. As a litre is many times the capacity of the system, any mixed fluid will be expelled at an early stage.

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 depressed in short movements until it becomes firm, this being a sign that the pads are now pressed against the discs.

This is also a good opportunity to remove and clean the ABS sensors, as they attract quite a lot of metallic debris from the discs, which can cause intermittent warning lamp operation. Just remove the socket screw (5 mm hex key) lever the sensor out of the hub, clean away any debris from the metal pole pieces on one side of the tip (I use a fine wire brush with brass bristles) and wipe with a rag moistened in white spirit etc. Lightly grease the fixing bolt and the body of the sensor before replacement, as these often-forgotten items can become rusted into the hub casting.

As you will see, brake bleeding is not a difficult task, but it must be done properly, as otherwise your life – and those of others – could be at risk.