Brake fluid flush: how often?
 

I just bought a 1987 S4 that has been neglected for a while (2500 miles in the past 5 years), so a brake fluid flush is one of the no-brainers.

I have used a shop in Fresno for a long time with my 944, and they always want me to flush the brake fluid every two years. Is this sound advice, or are they just inflating my tune-up bills?

IMHO...they got it right.

Brake fluid absorbs moisture over time...which reduces its capabilities.

and rusts the steel components in the brake system causing seal failure.

2 years as already noted.

The latter. Every 5 years is plenty. Bleed a bit every year and you'll be fine indefinitely.

This is also a service that you'll never know if they actually did it unless you watch or they screw it up.

On my other cars, here is Minnesota where they salt the roads, lines rust from the outside and have failed in 20+ yo cars. I can't believe from the inside is any issue on a car that's not driven in harsh conditions..

Porsche recommends every 2 years in the WSM.

If your driving the car hard, doing some track days etc this interval would be reduced dramtically.

Most of the time the fluid is black or dark brown after 2 years, you can tell when it has been flushed.

It needs to be done unless you enjoy replacing brake parts.

As a minimum , at least every 2 years ! Or before every track day .

I make a habit of flushing out the caliper's volume of fluid before each track weekend, and after it...its convenient to do via gravity bleed as the good tires go on..and then as they come off.

Left on it's own, the junked fluid down low, eventually percolates up high all through the system.

Keep it movin south steadily if you use it that hard.


Maybe im too anal about it..but I had boiled fluid once..did not like it...and it was old ancient fluid.

Every two years is a good number for most cars. Track or mountain driving shortens the interval...

I hate replacing brake parts.

I've never replaced one that I thought might be bad due to old fluid. Many frozen calipers. Zero leaking pistons.

I have replaced many that were due to neglecting fluid changes.

If the car is tracked and using ATE super blue, best pracice is every 6 months, correct?

A sage old mechanic once reminded me that the hydraulics wil last indefinately if the hydraulic fluid is kept clean. For brake hydraulics, this means flushing accumulated moisture from the fluid in the system. The moisture that accumulates is a problem in two ways.

-- First is that the boiling point of the fluid drops preciptously as moisture accumulates. For aggressive drivers, that might mean a sudden and often catastrophic loss of brakes when caliper temps reach that lowered boiling point. Seems that steam pockets anywhere in the system are quite compressible, so the 'hydraulic' nature of the system is compromised whenever there is vapor. Remember that the system has virtually no static pressure when the pedal is up, so moisture will boil and make pretty large-volume bubbles. If the total volume of the bubbles in the system exceeds the swept volume of the applicable section of the master cylinder, your pedal motion will not be adquate to recompress the bubles to liquid, and you will have no brakes. Sometimes fast pumping of the pedal will give you some partial recovery, but you'll still be hunting for some Charmin after one of these episodes.

-- The second and certainly more subtle penalty from moisture in the hydraulic fluid is corrosion of the metal parts in the system. Moisture adds the oxygen that's otherwise missing in the system, allowing oxidation (rusting) of steel brake lines, steel master cylinder bores, and rusting o steel components in calipers, connecting hoses, etc. Further, metal flakes that break away from corroded areas are abrasive at best, and will damage caliper seals and master cylinder piston cup seals when they happen to lodge there.


The rate of moisture accumulation is a variable, affected by local relative humidity, and slao oddly by how many times the temperature in the fluid is cycled. The factory suggests a period of no more than 2 year between flushes to protect against the effects of the second problem. Almost all racing groups recomend no more than 1 year, and many will not let you pass tech inspection with fluid more than 6 months in the car, looking out for the effects of the first problem. So what should your interval be? Depends... Live in a dry climate and never drive the car? Two years might be fine. For the rest of us, every year is safer. If you do DE days, autocrosses or other track days, you already know that even new dry off-the-shelf DOT-4 fluids can suffer on hard drives, so you've already replaced with ATE Super Blue/Gold or Motul fluids with higher dry boiling points, and you change them at much more frequent intervals. Should us casual street drivers use a competition fluid? As glamorous as it might seem, competition fluids tend to lose their advantage more quickly as moisture accumulates. They quickly lose their advantages as moisture builds up, so by six months or so they are likely to be no better than an common off-the-shelf fluid is at the same age.

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Just a year or two after I bought my car, I swapped in ATE Super Blue fluid. Seeing the color change to blue was a great indicator that I had pushed the old fluid out. Or so it seemed... The next change was to Gold, so I know when all the blue-colored fluid was gone I was done. I pushed two whole liters of gold fluid through and there were still a few tinges of blue in the flush tube. New fluid mixed with old as I was flushing, and the color was telling me that I was just looking at dilution rather than replacement. Just a heads-up for when you do it next, and that may also help you decide the flush intervals that are best for you and the location and driving circumstances your car enjoys.

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Mine gets replaced every year or so, in conjunction with the annual oil change. I live in a coastal desert climate, seldom if ever drive in the rain, and I can easily justify a couple liters of Castrol LMA fluid for the project, in concert with inspection/cleanings of other under-car components and brakes. A few times I've done the flush with the calipers unbolted so I can roll them around and get any possible metal contamination pushed out the bleed ports. With the annual changes since I've owned the car, nothing has ever been visible in the clear flush tubing. So I've not been as religious about that the last couple flushes.

In the past we've held flush-fests at my house for those interested. Mine will be due again in the spring. Maybe a group session will happen again then, before the weather gets hot again. If you are overdue now though, don't wait for me. I just put gas in the car a couple weekends ago, filling from about a third of a tank. Last gas fill was in June, so you get an idea how much I've been home and driving the car. 'Not enough' doesn't begin to tell the story. :)

If running standard DOT 3 or 4 fluid 2 years is OK.

If you are running high performance DOT 4 or 5.1 fluid then a MAX of 1 year.
I normally change mine every 6 months of so.

I have also been noticing more and more master cylinders failing from a build up of corrosion in the bore from a lack of fluid changes.