Howdy, Stranger!

It looks like you're new here. If you want to get involved, click one of these buttons!

Howdy, Stranger!

It looks like you're new here. If you want to get involved, click one of these buttons!

Did you get a great deal? Let us know in the Values & Prices Paid section!
Meet your fellow owners in our Owners Clubs

Brake Fluid Flushing-A Needed Service?

I try to flush my brake system once per year. This
means replacing your old fluid with new. I use a
bladder-equipped (keeps air separate from the brake
fluid so that brakes aren't dangerously "spongey")
pressure bleeder with individual fitting for my
vehicle. Some cars need a special device to flush
the ABS system section during this operation. This
operation has really reduced wheel cylinder/master
cylinder repairs on my vehicles over the years,
and the brakes seem to operate better, as well. I'm
in real estate sales, and do a lot of short-haul,
stop-and-go driving. You all should see the color
of the fluid if you go, say, longer than 2 years
between flushing. The DOT 3/4 (glycol based) fluids
tend to attract moisture, and tend to thus boil at
a lower temperature--causing possible braking
problems or failure when under severe use, e.g.,
down a steep grade, etc--corrode surfaces they
contact(important-can seriously "pit"/damage master
and wheel cylinders!) when moisture-laden; also
seriously damage painted surfaces they might drip
on. I understand that for perhaps $20 or less per
car, manufacturers could use silicone-based brake
fluid systems, which attract less moisture, are
less corrosive, less damaging to paint, boil at a
much higher temperature (safer in heavy braking
situations). The bottom line: there would be
likely longer-lasting vehicles, fewer repairs,
fewer accidents, and quite likely many, many, many
dollars saved in many ways. Why don't we demand
that this be done!!!?? I'd pay the extra $$,
wouldn't you?


  • alcanalcan Posts: 2,550
    Silicone brake fluid does not have a higher boiling point than "dry" (less than 3% moisture) glycol fluid. Also, silicone fluid tends to aerate when forced through small orifices. Like the ones in every ABS pressure modulator. There are currently ZERO car manufacturers who recommend the use of silicone brake fluid in ABS equipped vehicles..
  • To alcan: I'm wondering, then why do brake fluid labels of glycol-based fluids indicate in temperature numbers a lower boiling point than that indicated on the silicone fluid containers? And ,given the glycol propensity to attract/absorb moisture, how realistic in a safety-related standard is it to compare dry boiling points?
    Also, my understanding is that rubber parts in most all stock brake sytems are the "seal swell" type and thus incompatible with silicone fluid-those rubber parts would need to be replaced with appropriate ones before using silicone fluid. This is another reason that mfrs would not recommend silicone fluid in basically all stock brake systems. I hadn't seen information on silicone fluid aeration! Maybe it's time to develop another fluid altogether!

    What do you think about periodic flushing of brake fluid?

  • butch11butch11 Posts: 153
    This may not get rid of all of the fluid in the system but it works for me. Before changing front disk brake pads-I take a turkey baster and suck out all of the fluid in the master cylinder supply bowl and then replace it with new fluid. When you change the disk brake pads-open the drain spigot and then push the piston back in. this squirts all of the fluid that was in the caliper into your oil change pan-not ground please. Hopefully the stuff in the metal lines does not have too much water in it.

    Just make certain to not let air back into the piston-use a big C clamp to push the piston in-easier than prying it back.
  • Butch 11,
    I read or heard somewhere from a mechanic that your method, common practice before, could possibly damage the newer(ABS?) systems by forcing debris back up the lines. Has anyone else heard something on this?
  • Butch 11,
    'Forgot to thank you for pointing out that fluids from car maintenance should be properly collected/disposed of!!!!! In the San Gabriel Valley area of Southern California, we have regular hazardous waste "round-ups" in various locations, as well as hotlines for such info in between events. Hopefully we can move toward an even better set-up for such disposal, so that the inconvenience involved doesn't lead to more poisoning of people and things.
  • butch11butch11 Posts: 153
    By opening the small drain valve on the back of the caliper, when you push the disk brake piston back into the caliper to make room for the new pad, the fluid is pushed out of the drain valve instead of back into the brake system.

    Some people used to use the C clamp to push the fluid back into the brake system-sometimes causing the master cylinder to overflow. The real danger is getting air into the system-you can buy or make a device to keep air out of the system-use a small glass jar, 3/8" id clear flexible hose, fill it with fluid, fluid in jar covering end of the line and this will keep air out of the system. In my experience it is only necessary to do this when replacing the caliper and filling an empty disk brake piston.
  • alcanalcan Posts: 2,550
    A lot of misinformation exists regarding Polyalkylene Glycol Ether (glycol) and Silicum-based Polymer (silicone) brake fluids. All brake fluids must meet U.S. federal standard #116. Under this standard are Department of Transportation (DOT) minimal specifications DOT 3, 4, and 5.1 for glycol based fluids, and DOT 5 for silicone fluids.

    First, it should be understood that water will probably eventually find it's way into any brake hydraulic system through microscopic pores in flex hoses and master cylinder reservoir diaphragms, at the rate of 2-3% annually. Glycol fluids are hygroscopic (they absorb moisture) for a reason. Any moisture contamination in the system will tend to be dispersed throughout the system, minimizing water concentration at any specific location.

    Silicone fluids, being non-hygroscopic, will not disperse water, which can cause high concentrations of water at the lowest points of the system such as the brake calipers or low points in lines (water is heavier than silicone fluid and will collect at the low points). This presents 2 problems: localized higher corrosion rates; and more chance of water collecting and boiling in the calipers under heavy braking, causing gassing and pedal fade. Silicone fluids also contain about 3X as much dissolved air as glycol fluids and are about 2X more compressible, which may cause the characteristic spongy pedal feel associated with their use, cause aeration when forced through small orifices, and make bleeding more difficult.

    Regarding seal swelling, some early silicone formulations had an incompatibility problem with glycol-designed seals but that has been corrected in most current formulations. If glycol-designed seals were designed to swell with use, then every rebuilt caliper/wheel cylinder would leak when first installed.

    Currently, Harley Davidson is the only U.S. manufacturer to specify silicone fluid, and that's for paint damage concerns. None recommend silicone fluid in ABS equipped vehicles.

    FLUID TYPE DRY - boiling point - WET
    @ 3% water
    DOT 3 glycol 401 F 284 F
    DOT 4 glycol 446 F 311 F
    DOT 5 silicone 500 F (humidified)=356 F
    DOT 5.1 glycol 518 F 375 F

    DOT 3 represents the MINIMUM standard for disc brake fluid boiling points. I recommend Valvoline SynPower @ 513 F & 333 F, and Ford Heavy Duty glycol fluids to my customers.

    For anyone still wanting to switch from glycol to silicone fluid, be advised that the two do not mix, and will cause coagulation. The system must first be thoroughly flushed with denatured alcohol or methyl hydrate.

    The bottom line is simple. Any brake hydraulic system is subject to water contamination. Glycol filled systems should be flushed every 2-3 years (this is an issue of time and relative humidity, not accumulated mileage). Silicone filled systems should be flushed annually.

    Sorry for the textbook, folks, but there it is.
  • Alcan, thanks for your thorough-yet-succinct words on perhaps the most-neglected (and critically safety-related) vehicle maintenance area! If a motor wears out due to too-infrequent oil changes, it's not likely to endanger people on the road, but brake failure obviously can affect dozens of vehicle occupants surrounding another vehicle where its brakes fail! I'm sure that I knew more about the subject than the average driver, and yet I learned a lot from your post (#7). Your post should be required reading for anyone who has responsibility for vehicle upkeep!
  • Enough Said!
  • 210delray210delray Posts: 4,722
    Thanks for the advice in the other brake system flush topic (now read-only).
  • alcanalcan Posts: 2,550
    You're welcome. I could feel Bruce and Pat coming to chase us away (LOL).
  • 0patience0patience Oregon CoastPosts: 1,712
    One thing to add to Alcan's info about switching from DOT 3 or 4 to silicone, it use to be a practice in the late 70s and early 80s (some European vehicles came with silicone brake fluid, then) to switch from DOT3 brake fluid to silicone, more often than not, there would be wheel cylinder seal and caliper seal failure almost immediately because of it. Even systems that were thoroughly flushed. Usually in order to switch, the systems had to have new compnents installed. Rarely were any people who requested the switch happy in doing so.
This discussion has been closed.