Brake Fluid Change ;
3 methods, pressure, vacuum or pedal.
Pedal is best avoided if possible, although it is "free" as you don`t have to buy anything. The idea is (for pedal) to loosen the caliper bleed screw then press the brake pedal to pump the fluid through. Problem is that the pedal will go down further than normal. I have heard from several places that the pistons in the master and slave cylinder go in further than normal, ie into "unused" stroke, and this can rip the seals used to going just a certain smoothed bore distance.
Pressure and vacuum are fairly similar.
Pressure;
A bottle of fluid attaches by a hose to the brake fluid reservior. There is a second hose which goes to a pressure source, often a hand pump (like a garden sprayer) or more often, to your spare tyre. Important. You MUST have the spare at a very very low 10-14psi, the 46ish psi of the fully inflated spare will damage the brake system.
Basically, you open the valve on the "bottle", then undo the brake caliper screw. The pressure from the tyre fills the bottle with air, which forces the new brake fluid into the reservior, which forces the old fluid out the brake lines.
If I recall the fronts should be done, then the rears.
Finally, watch out that the bottle does not empty and just air get forced into the system, as when it (air) reaches the master/slave cylinders it gets problematic.
Vacuum;
Similar. This time a pump gets connected to a hose which connects to the brake caliper bleed screw. You open the brake fluid reservior, then pump away. The fluid gets sucked out by the wheels, and you add new fluid into the reservior as it dips. When the fluid goes clear, that wheel is done. Repeat the other wheels.
All of the above are much easier with a friend to help.
The Bentley manual covers it well, there is also information on the BMWE34 website
here
If I recall then a pressure kit from the likes of your average auto store is about $30 (£20) which uses the spare tyre as a source.
As for the fluid, the DOT rating (from memory, sorry if I`m wrong, but I`m sure I`ll be corrected !!!), there is now only numbers 3,4,5 available. The rating is how hygroscopic the fluid is, ie how quickly it will absorb water from the enviroment. The higher the rating, (eg Dot 5 racing style fluid) this will provide the best performance, but will quickly deteriorate in comparison to the DOT 3 and 4.
I have a vague notion that the higher numbers can also absorb more heat due to their formula, ie higher boiling point (assuming no water absorbed due to fluid age), but that may be incorrect.
I believe the M5 needs DOT 4 as standard, but DOT 5 won`t harm anything. Whatever you use, the fluid will need changing at the least once a year as part of routine maintenance.
I don`t think that you will need DOT 5 unless you are very very serious about huge braking and hit the track regularly.
Just to say, in case you didn`t know, the "fade" that brakes have is mainly due to the fluid boiling. (assuming your pads aren`t cooked and the rotors broke!!)
What happens is that the fluid boils, producing gases. Gas compresses, so your braking force goes to compressing this gas instead of forcing the pads onto the brakes rotors.
Brake fluid is hygroscopic as I said, it absorbs water. When this happens, it has the effect of lowering the boiling point of the now contaminated brake fluid. It is easier to boil (steam!!) the fluid, and hence the fade again as this gets compresses under braking.
Don`t get too hung about the fluid though. A yearly change is OK for 99.9% of road users as opposed to racers. There was an excellent link on the E39 area recently, from a few brake experts.
They correctly pointed out that the brake system must be considered as a whole. The pedal presses, the master and slave cylinders take a little slack, the fluid tries to compress, the brake lines expand a little, the pads get pressed once the slack in the calipers gets taken up. Finally friction starts to slow the disk, and finally the tyre contact patch actually starts to exert braking force against the road, and this is what stops you. So all parts must be in working order for the brakes to be effective.
Phew !!
Ivan.
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