AUDI A4 | B5 | 8D 1996 - 2001 FAQ Frequently Asked Questions and Common Problems Thread Collection

[pac1085]

If you experience low RPM bucking, etc. you need a newer MAF. Part # 037 906 461C

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[MyaLover]

• Cleaning with pics

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[Yippers®]

• http://www.vw-store.com/Tools-Hardware.htm

[A4SilverT]

• Getting radio codes

[A4SilverT]

DISCLAIMER: the following information has been culled from various locations on the web. I'm not an expert chemist and can't independently evaluate the data that follows, but it seems consistent over a number of sites.

There are three U.S. Department of Transportation (DOT) general specifications of brake fluid, DOT3, DOT4, and DOT5, based on the boiling point of each. A high boiling point is very desireable in a brake fluid because it allows more hard breaking (pressure creates heat) before the fluid boils. Boiling fluid leads to a loss of brake performance. The technical explanation for this is that boiling leads to cavitation within the system and loss of braking pressure.

The first two specifications of brake fluid are glycol-based while DOT5 is silicone-based. Glycol-based fluids absorb water when exposed to the air. The more water, the lower the boiling point of the fluid, and the worse brake performance.

The longer the glycol-based brake fluid stays in the vehicle, the more water it will absorb. The general recommendation is somewhere around 2 years for flushing and refilling.

A silicone-based fluid won't absorb water, but can still be contaminated by it; water contamination will form globules that will sink to the lowest point in the system, i.e. the calipers. The water globules would boil at 100 degrees centigrade, leading to vapor lock and possible brake failure. For those who use silicone fluid to race, it has been suggested that bleeding a bit of fluid from each wheel before the race would clear any water globules from the system.

However, some experts suggest that DOT5 should not be used in systems with antilock brakes or for racing. For example, Stoptech argues that because silicon-based brake fluid is more compressible, it can create a spongy pedal feel that provides poor braking feedback due to the imprecise modulation (feeling small differences).

Because of those problems with silicone, DOT5.1, a glycol-based fluid that was designed to meet DOT5 performance standards, was invented. It equals DOT5 boiling points without the contamination and compressibility problems of silicon-based fluid.

Here are the boiling points of the various fluids in degrees Centigrade:

DOT3

Uncontaminated 205C
Contaminated 3% 140C

DOT4

Uncontaminated 230C
Contaminated 3% 155C

DOT5 and DOT 5.1

Uncontaminated 260C
Contaminated 3% 180C

It's worth noting that these are minimum standards. Within each standard, different brands of fluid may perform better than others.

The single most common brake system failure caused by a contaminant is swelling of the rubber components (piston seals etc.) due to the introduction of petroleum based products (motor oil, power steering fluid, mineral oil etc.) A small amount is enough to do major damage. Flushing with mineral spirits is enough to cause a complete system failure in a short time. Flushing with alcohol also causes problems. Brake systems should be flushed only with DOT3 or 4 or 5.1.

If silicone is introduced into an older brake system, the silicone will latch onto the sludge generated by gradual component deterioration, creating a gelatin-like goop which will attract more crud and eventually plug up metering orifices or cause pistons to stick. If you have already changed to DOT 5, don't change back. Silicone is very tenacious stuff and you will never get it all out of your system. Just change the fluid regularly.

If you do a search for "racing brake fluid" you'll find lots of things advertised as being for racing use. But racing brake fluid is not a specification.

If you look closely at what is advertised, some of it is listed as only meeting the DOT3 specification. Most of the rest is DOT4. As of this writing, only AP Racing meets the DOT5.1 standard. Some "racing brake fluid" is advertised as not for use with clutch systems, making it useless for A4s with manual transmissions since the clutch and brake systems share the master cylinder.

"Racing brake fluid" ads talk about dry boiling point and wet boiling point, but it's not at all clear how that relates to DOT standards (uncontaminated, 3% contamination). That's the problem when manufacturers do their own testing.

Possibly some of the manufacturers are waiting for DOT5.1 certification of the their products, but until that happens, almost all the "racing brake fluid" I've found, including Motul RBF and Pentosin Racing, is DOT4.

[TristanP]

• http://forums.audiworld.com/a4/msgs/1993310.phtml