General Question

alexcsm's avatar

Do I need a brake flush?

Asked by alexcsm (4points) July 28th, 2009
6 responses
“Great Question” (1points)

Took my 2000 integra to Tires Plus for oil change. They said I needed a brake flush and showed a purple litmus strip. I have never heard of this procedure before. Is it needed??

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itsjustmatt's avatar

No, unless you’ve had excessive leakage of brake fluid and have had to constantly refill the reservoir. That might introduce contaminants and moisture, otherwise tell the mechanic to take a hike and that you won’t be back there.

Harp's avatar

It’s likely that you need the flush. Honda (who built your car) recommends a flush every 25–30K miles. You surely have way more than that. The problem is moisture absorption. The fluid contains a product which attracts water, and over time it will draw moisture in through pores in the hoses. Once the moisture content goes over 2%, the boiling point of the fluid drops from about 450º to about 290º, low enough to conceivably boil under heavy braking conditions.

There are several ways to test the condition of the fluid. The strips work by testing the anti-corrosion additives in the fluid for the presence of copper, which is an indication of useful life.

Different manufacturers have different guidelines for brake fluid maintenance. GM now uses brake lines that are supposed to eliminate moisture transfer, and a fluid that is supposedly more resistant to moisture, so they don’t call for flushes. Ford recommends a flush every 36K.

jonklein611's avatar

+1 for flushing at service intervals. As Harp says, it depends on your vehicle but it’s not hard to do and can end up saving a life!

itsjustmatt's avatar

@Harp That typical mechanic’s answer makes sense until you look at the physics (or just common sense) of it all. First, if there were any pores in the hoses no matter how tiny, you’d have brake fluid coming out of everyone of them everytime you hit the brakes. Second, it doesn’t make sense to think the system could ever soak up 2%!! volume of water but never leak a drop of fluid. Third, that boiling temperature is at atmospheric pressure. During braking the line pressure is at about 1200psi, depending on the car. At 1200psi, water doesn’t boil until 564º which is higher than the DOT spec of the brake fluid.

Harp's avatar

@itsjustmatt Water molecules are much, much smaller than those of the glycol-ethers used in most brake fluids. A pore in a hose or seal that would stop a glycol molecule looks like a barn door to water vapor.

In humid environments, that 2% water content can be reached in only one year of use. After a few years of use, 7–8% is not uncommon.

During braking the pressure is indeed high, but when the pressure is released the heat is still there and the water will vaporize in that low pressure environment.

itsjustmatt's avatar

@Harp I must agree with every thing you say, however, I’m still not convinced there is that much in leakage. I need more proof of this. What sort of experiment could we devise, so you could prove me wrong? Seriously :)

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