2006-2009 Audi RS4 Brake Fluid Type and Flush Interval

By Core Brake Parts Tech Team — Circleville, OH warehouse

The B7 Audi RS4 takes a DOT 4 glycol-based brake fluid, and Audi's maintenance schedules call for replacing brake fluid every 2 years regardless of mileage. On a car with this much brake, a low-viscosity DOT 4 that lists Audi's own reference numbers is the right choice for the street, and a higher-boiling DOT 4 is worth considering if the car sees track days.

Below is what the fluid has to do on an RS4, what the DOT numbers mean, how to tell your fluid is overdue, and what to do at the same time as a flush.

What Brake Fluid the Audi RS4 Uses

The B7 RS4 uses a conventional glycol-ether DOT 4 hydraulic system shared with its ABS and ESP stability control. Audi's genuine brake fluid is sold under the B 000 750 part-number family, and aftermarket fluids approved as equivalents list those Audi references on their data sheets. Pentosin DOT 4 LV, for example, lists Audi/Volkswagen B 000 750 M1, M2, M3, M7 and M9 as OE references and states it can be used in all vehicles that require DOT 4.

The "LV" stands for low viscosity. The federal DOT 4 standard allows up to 1,800 mm2/s of viscosity at -40 C, while Pentosin's DOT 4 LV is rated at 700 mm2/s or less at that temperature. Thinner cold fluid moves through the ABS and ESP valve block faster, which is why European makers moved to low-viscosity DOT 4 for cars with stability control.

Always confirm the fluid printed on your reservoir cap and in your owner's manual. If the cap says DOT 4, stay with a DOT 4 glycol fluid.

DOT 3, DOT 4 and DOT 5.1 Compared

Every brake fluid sold in the US must meet Federal Motor Vehicle Safety Standard 116. The two numbers that matter most are the dry boiling point (fresh fluid) and the wet boiling point (fluid that has absorbed water). Here are the federal minimums next to one DOT 4 LV product:

DOT 3, DOT 4 and DOT 5.1 are all glycol-based and can mix, but mixing drags the whole system down toward the weakest fluid. DOT 5 (no decimal) is silicone-based and does not belong in an RS4 hydraulic system.

Why the 2-Year Flush Interval Matters on This Car

Audi's published US maintenance schedules list "Replace brake fluid (all vehicles)" as a time-based item: every 2 years regardless of mileage. It is not a mileage item because glycol brake fluid absorbs moisture through hoses and seals whether the car is driven or stored. As water content climbs, the boiling point falls toward the wet rating in the table above.

That matters more on an RS4 than on an ordinary A4. The front brakes are large 365 mm floating rotors, and the car is capable of speeds that put a lot of heat into the calipers in a single hard stop. When fluid boils in the caliper it forms vapor, and vapor compresses. The result is a pedal that goes soft or sinks toward the floor right when you need it most. Many RS4s now spend a lot of time parked as weekend or collector cars, which is exactly the situation where owners skip the time-based flush.

If you track the car, treat 2 years as the maximum, not the target. Flush before a track weekend and check the fluid condition after.

Signs Your RS4 Brake Fluid Is Overdue

• Soft or long pedal after hard use that firms up again once the brakes cool. This is the classic sign of fluid boiling in the calipers.
• Dark fluid in the reservoir. Fresh DOT 4 is typically clear to amber; dark brown fluid has been in service a long time. Color alone is not a moisture test, so go by the date too.
• No record of a flush in the last 2 years. On a used RS4, assume it is due unless there is a receipt.
• Brake warning light or low reservoir level. A falling level usually means pad wear, but it can also mean a leak. Inspect before topping off.
• Corrosion concerns. Old fluid loses corrosion protection, which is hard on caliper pistons, ABS valves and the master cylinder over time.

A shop can test moisture content or boiling point with a meter, but on a car with this kind of brake system, a flush on schedule is cheap insurance.

How the Flush Is Done and What to Replace at the Same Time

A proper flush pushes fresh fluid through every caliper until clean fluid comes out of each bleeder. Most shops pressure-bleed from the reservoir. Because the RS4 has ABS and ESP, follow the bleed sequence and any scan-tool ABS bleed procedure in the factory service information rather than guessing the order. Keep the reservoir from running low during the job so air never enters the ABS unit. Tighten bleeder screws to the value in the factory service manual; they are small and strip easily.

This is a moderate DIY job with a pressure bleeder and a second set of eyes, and a routine job for a shop. The best time to do it is when you are already into the brakes:

• New pads. For RS4s on the factory cast-iron rotors, we carry EBC Redstuff ceramic low-dust front pads for the RS4 for street driving, EBC Yellowstuff street and track front pads for spirited use, and EBC Bluestuff NDX front pads for track days.
• Rotors. If the stock 365 mm rotors are worn or cracked, GiroDisc 380 mm slotted front rotors with caliper spacers are a larger, lighter replacement; GiroDisc lists its pair at about 22.6 lb versus 25.5 lb for stock. For the rear there are GiroDisc A2-100 slotted rear rotors and a Brembo GT rear big brake kit for the RS4.
• Brake hoses. Aged rubber hoses swell under pressure. See our brake lines and brake hoses.

Any time a caliper or line is opened for a parts swap, the system needs a full bleed anyway, so combine the two jobs.

Street Fluid vs Track Fluid for the RS4

For a street-driven RS4, a quality DOT 4 LV that lists Audi's references is the best match: it clears the DOT 4 boiling points with margin and keeps cold-weather ABS and ESP response where Audi designed it. For regular track use, owners often step up to a higher dry-boiling-point DOT 4 racing fluid. The trade-off is that many high-boiling fluids are thicker when cold and need changing more often. Whatever you choose, do not top off with a partial bottle of something else; flush the system completely to the new fluid.

Frequently Asked Questions

What type of brake fluid does a 2006-2009 Audi RS4 take?

A glycol-based DOT 4 fluid. Low-viscosity DOT 4 that lists Audi's B 000 750 references is the closest match to factory fill. Confirm on the reservoir cap.

How often should I change brake fluid on my RS4?

Audi's maintenance schedules say every 2 years regardless of mileage. Track cars should be flushed more often, typically before events.

Can I use DOT 3 in my RS4?

No. DOT 3 has lower federal minimum boiling points than DOT 4. Use the DOT 4 fluid specified for the car.

Can I use DOT 5.1 instead of DOT 4?

DOT 5.1 is glycol-based and compatible with DOT 4 systems, but it is not what Audi specifies. Never use silicone DOT 5.

Do I need a scan tool to bleed the RS4 brakes?

A routine pressure-bleed flush usually does not need one, but if air gets into the ABS/ESP unit, the factory procedure uses a scan tool to cycle the valves. Follow the service information.

Is it bad to leave old brake fluid in a car that sits a lot?

Yes. Brake fluid absorbs moisture over time, not miles, which is why Audi lists it as a time-based item. Stored cars still need the 2-year flush.

Sources

1. Audi Maintenance Schedule, All Models (NHTSA filing) - brake fluid replacement every 2 years regardless of mileage, all vehicles
2. 49 CFR 571.116 (FMVSS 116), Cornell LII - DOT 3, 4 and 5.1 dry and wet boiling points and -40 C viscosity limits
3. Pentosin DOT 4 LV product data sheet (CRP Automotive) - boiling points, viscosity, Audi B 000 750 OE references
4. GiroDisc B7 RS4 380 mm front rotors - stock 365 mm front rotor size and rotor weights