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Let's assume I have a car with 30k miles whose service manual does not specify any interval for changing brake fluid, and the fluid is still transparent with no visible contaminants.

Is it feasible for me to use a turkey baster to suck out 75% of the fluid in the master cylinder reservoir, replace the extracted fluid with the same amount of new brake fluid, then repeat that process every so many miles miles until a satisfactory number of cycles are complete, effectively replacing most of the fluid in the system? If so, every how many miles should I do the partial replacement, and for how many cycles?

The idea is that as the brakes are used over those miles, the fluid is mixed and routed through the entire system. That mix is then diluted of old fluid even further when the process is repeated again. After enough cycles, the old fluid (along with its contaminants) would be diluted so much that it is effectively clean and has a low enough contaminant concentration to be satisfactory.

I chose 75% for the amount to be replaced because I didn't want to suck the reservoir dry and possibly introduce air into the system, which would make it necessary for me to bleed the brake system. That's what I'm trying to avoid having to do.

The one thing I can think of that could thwart this idea is that the fluid in the master cylinder reservoir doesn't completely cycle through the system in any reasonable number of miles (or ever). Is my idea a valid one, or am I mislead?

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    Good question! This would have been much easier to test back when Super Blue was still legal!
    – MooseLucifer
    Commented Sep 20, 2017 at 22:49
  • @MooseLucifer Wait, Super blue is illegal?! That stuff was awesome. (Of course it's been 15 years since I had use for it)
    – 3Dave
    Commented Sep 20, 2017 at 22:54
  • @david lively yep, took the fmvss a little over a decade to realize that their own bylaws said all dot4 in the states has to be amber, for whatever reason.
    – MooseLucifer
    Commented Sep 20, 2017 at 23:01
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    if it's ATE Super Blue, I've searched, there's a mention of a "Gold type 200" that according to the description is the same,sans blue dye
    – user16973
    Commented Sep 21, 2017 at 4:36
  • With the Super Blue they had another spec of fluid that was the same just not dyed blue. Plenty used it so they could switch between them and easily tell when it was flushed through with fresh fluid. A good idea. Pity it is no longer available.
    – Kickstart
    Commented Jan 6, 2020 at 9:05

3 Answers 3

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There's one slight problem with your theory. The fluid which goes out to the brake cylinders/calipers stays out there. It will never come back to the master cylinder. You have to do something to remove this fluid. The only way is to flush it out.

The biggest issue with all of this is the brake fluid which never comes back and stays in the lines will absorb water over time. The water will then act to cause corrosion on the steel brake lines. Sooner or later it will rust through and have issues. If you don't flush them, it will have issues sooner or later.

As an aside, the normal change interval for brake fluid is two years, regardless of mileage. After that it will usually start to change color. The darker it gets, the more water it has absorbed.

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  • That's exactly what I feared, the fluid not being totally cycled through. Thanks Paul!
    – Carlo David
    Commented Sep 20, 2017 at 22:39
  • And further the fluid in the caliper bores is the "most" contaminated - as it has the most exposure to the elements and moisture. Not to mention wicked heat. The piston seals are less than perfect, and constantly cycled. Still no need to flush; baster plus good bleed does a proper job.
    – SteveRacer
    Commented Sep 21, 2017 at 3:05
  • There is also a chance that if the fluid in the master cyl drops below a certain point, an air-lock may be introduced which would have to be bled out anyway.
    – Steve Matthews
    Commented Mar 11, 2019 at 12:13
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I sucked out ALL the old green coloured fluid from the reservoir and replaced with new clear Toyota Dot 3, without bleeding. After just a couple of miles driving, the clear fluid in the reservoir turned slightly greenish. The next day I drove 20miles and observed that the fluid had turned even more greenish. These are evidence that fluid in the reservoir is circulated and mixed with old fluid in the lines below. The idea that fluid in the lines are stationary is not correct because the pumping action as the brakes are depressed and released generate hydraulic movement that circulate the fluid throughout the entire system. Even when the vehicle is not being used, diffusion alone will eventually mix the old and new fluid together although the process will take longer.

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  • it's more likely that you didn't get all the fluid out of the master cylinder. Obviously old fluid was somewhere or it would never turn green.
    – John Lord
    Commented Nov 30, 2020 at 21:44
  • I flushed the reservoir that was 4% water.. added dot3 that showed <1%. A few months later its back to 4% so it must be circulating but not sure how deep. So I did it again and in a few months I'll check back to see again.
    – radtek
    Commented Dec 6, 2022 at 5:24
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Your method is alright, don't worry, it will mix, but you have to drive a few hundred miles before next change.

I did it myself long time ago with Brake Fluid Tester showing my old Brake Fluid was at least at 4 % of water contamination.

I tested the new bottle DOT-4 it was 1% (inside the Bottle) then I sucked up all Reservoir fluid (all of it ) and then poured the new DOT-4. After 1 week of driving (high-ways and city traffic), I re-tested my reservoir, guess what: it was 3%. This is evidence that the fluid will be mixed. I then repeated the process until I got 1% of water in the reservoir,

By the way, don't worry about air if you sucked all the reservoir, because the master Cylinder just below it will be full of fluid and no way the air enters it. When you pour the new fluid into the reservoir, the air in the reservoir will be expelled automatically.

I have suffered from old Fluid where I lose brake sometimes, especially in the traffic in summer, but now it is alright.

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  • Did you understand the other answer? The fluid in the pipes is basically stationary...
    – Solar Mike
    Commented Mar 12, 2019 at 16:01
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    actually i think you did not understand my comment, i did the test my self and i proved it with numbers, please read it again, also who said it is stationary, it is fluid when it gets hot at caliper it will go up and the cold fluid will replace it it will mix, simple physics.
    – majed9r
    Commented Mar 13, 2019 at 8:47
  • If you evaluate the volume of liquid in the pipe and using the cubical coefficient of expansion and delta T work out how much fluid will move then you might have evidence to support your answer... Or you may not...
    – Solar Mike
    Commented Mar 13, 2019 at 9:11
  • i know the volume of the fluid in Brake circuits of my vehicle, but i don't need to make it harder on me neither any one here, i just did an experiment on my own vehicle 3 years ago, i provided the numbers i got from tester in the reservoir 4% then 3 % then 1 %, accordingly i got much much better grip , i just want to share it with every body, and i don't care if they believed. by the way our main dealerships don't make service through bleeding, they only follow this procedure.
    – majed9r
    Commented Mar 13, 2019 at 9:27
  • Sorry, good dealerships and garages use a machine that forces new fluid through from master cylinder all the way to each and every caliper - I'm surprised you don't know this...
    – Solar Mike
    Commented Mar 13, 2019 at 9:40

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