Q: Is it necessary to flush the brake fluid after 20,000 miles, as my Mercedes dealer recommends?

A: I recommend this routine service for every motor vehicle. Historically, domestic carmakers haven't listed this as routine service for their vehicles, but it has been, and continues to be, a recommended service for most European and performance cars. In fact, race cars are prepared for every race with fresh brake fluid.

Why? Moisture. Hydraulic brake fluid is a glycol ether-based fluid that is hydroscopic, meaning it has an affinity for attracting water. Even the smallest amount of moisture contamination in brake fluid can significantly reduce its boiling point.

This is critical because the characteristic of brake fluid that makes it appropriate for use in hydraulic systems is its incompressibility. There is no loss of force transmitted from the brake pedal through the master cylinder to the caliper or wheel cylinder. When fluid is contaminated with moisture, temperatures reached in the caliper or wheel cylinder can boil the moisture, generating gas bubbles in the fluid which are, of course, compressible. The symptom of this is a soft, somewhat unpredictable brake pedal — not particularly confidence-building when trying to stop the vehicle.

Since some percentage of moisture absorption is normal over time, a periodic flushing and bleeding of the hydraulic system to replace any contaminated brake fluid is a very good maintenance practice. In addition, removing moisture reduces rust and corrosion forming in the brake system.

Q: I have 125,000 miles on my 2001 Cadillac Eldorado. The check engine light came on a few weeks ago. The manual said it might be the gas cap. If the light is not flashing it usually is nothing serious. The dealer highly recommended a scan to determine the problem. The scan tool pulled up DTC P0741, which has to do with the overdrive not kicking in at higher speeds. They said a transmission overhaul would cost between $5,500 and $6,000 but recommended I not have this work done because of the cost, the vehicle's age, that it will have no real effect my routine driving and is not a safety issue. I was really impressed that the dealer did not push for me to have this done, but wonder what your opinion is.

A: I agree with your opinion of the dealer and the dealer's recommendation. According to my ALLDATA database, the P0741 fault code is set when the transmission torque converter clutch slip speed exceeds its limits for a specific engine torque for more than five seconds. The cause can be as simple as contaminated or low fluid level, or that the transmission is operating in the hot mode, which prevents engagement of overdrive.

The dealer is correct, there is no safety issue and you can continue to drive the vehicle. But I would suggest the KISS approach — add a half-can of SeaFoam Trans-Tune to the transmission fluid, drive the car for several weeks, then have the transmission fluid and filter changed.

Q: I have a 2009 Chevy Malibu with the four-cylinder engine and 61,000 miles. After driving for about 20 minutes my RPMs rev up and the car surges then goes back to normal but the cycle keeps up. Please advise.

A: Because there could be so many possibilities, start by checking fluid levels in the engine and transmission. High engine or transmission operating temperatures or low transmission fluid level could be the cause. Is the vehicle up-to-date on maintenance? A restricted air cleaner, exhaust system or a simple vacuum leak could cause this characteristic.

Have a scan tool identify any stored fault codes in the ECM or TCM. This may identify a problem with the TCC — torque converter clutch — operation. If this occurs more often in warm weather at speeds in the 60- to 65-mph range, check with your dealer. GM issued powertrain software updates that may address your issue.

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ABOUT THE WRITER

Paul Brand, author of "How to Repair Your Car," is an automotive troubleshooter, driving instructor and former race-car driver. Readers may write to him at: Star Tribune, 425 Portland Ave. S., Minneapolis, Minn., 55488 or via email at paulbrand@startribune.com. Please explain the problem in detail and include a daytime phone number. Because of the volume of mail, we cannot provide personal replies.

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