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Thread: Flushing Hydrulic Fluid

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    Flushing Hydrulic Fluid

    I have to replace both master cylinders and while I'm at it I'd like to flush out both systems and replace the old fluid with DOT 5. What is the best way to flush it out? I'm thinking that I can use compressed air to blow it out. That means disconnecting the lines to all the cylinders and then treating the cylinders separately. Is that the way to go or is there a better way?

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    Yoda Geo Hahn's Avatar
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    Re: Flushing Hydrulic Fluid

    You'll get various opinions on this.

    I just sucked out what was in the reservoir, filled it with DOT5, then pedal-pumped and bled (one corner at a time) until the new stuff was coming out.

    That was 18 years ago in the case of the TR3A.

    How nice or nasty is the fluid in there now? If it looks pretty good (amber, not black) then that may help in your decision.

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    Jedi Knight Got_All_4's Avatar
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    Re: Flushing Hydrulic Fluid

    I did exactly what you are planing to do. Don't remember where the info came from but it was strongly suggested that it be flushed out with isopropyl alcohol. Took a couple of bottles and pumped them through a couple of times by saving and reusing it. Then took a couple of fresh bottles (quart each) and did the same with each one of them until the alcohol ran clean. To dry out the system I had an old reservoir cap so I drilled a hole in it to accept a air nipple. Then screwed it on top of the reservoir and ran compressed air through it for about a half hour through each individual bleeder screws. That was a couple of years ago and like GEO says no problems. Also no worries!

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    Re: Flushing Hydrulic Fluid

    I am far from an expert, but after flushing with alcohol it seems care needs to be taken to ensure all solvent is out. Otherwise the resulting fresh fill of fluid will be thinned.

    Can any chemists in the group talk about what happens to "dissolved" hydraulic fluid - meaning fluid that the alcohol has dissolved/thinned, but still remains in the system. The air will evaporate the alcohol, but will it evaporate the "dissolved" hydraulic fluid?

    Got_All_4, I am not doubting or debating what you did in the slightest. Just curious about the cautions one must take.
    Mike
    66 TR4A

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    Yoda Geo Hahn's Avatar
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    Re: Flushing Hydrulic Fluid

    Both methods will leave some old fluid in the system though much less so in the case of the alcohol flush.

    All I can say is that (on 3 cars in my case) it hasn't been a problem.

    I have never disassembled a caliper after doing this - I have replaced pistons and seals through the years but never had to split a caliper open - so I cannot say what is in there.

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    Re: Flushing Hydrulic Fluid

    I am told that DOT5 will mix with DOT3 and 4 so even if the old brake fluid is not completely flushed with air there shouldn't be a problem if a little of the old fluid remains.

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    Re: Flushing Hydrulic Fluid

    Dot 5 won’t mix, but as Geo has seen some residual Dot3 or 4 hasn’t been an issue. I used denatured alcohol and air dried, worked on 2 cars and I currently converting a third. In my case I changed out the rear cylinders at the same time - they are pretty cheap and it avoided any worries of hurting the seals.
    Randy
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    64 TR4 - got another one!

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    Re: Flushing Hydrulic Fluid

    This board has had some rather lengthy threads on DOT-5 and DOT-5 conversions. In addition to this thread you should search for some of the older ones. I don't wish to repeat all of their content here but I will make the following statements. These are based on personal experience as well as what I have seen with others first hand.



    • DOT-5 will NOT mix with any other brake fluids. Period. Clean all old fluid out of the pipes.
    • Yes you can flush DOT-3/-4 out the lines with isopropyl. Do it multiple times and blow the lines dry between alcohol rinses.
    • Do NOT use DOT-5 with seals that have already seen service with DOT-3/-4. Replace ALL the seals with new and include with that the rubber flex hoses, and the internals of things like brake servos and PDWAs.
    • If you happen to be working on a car with a hydraulic brake light switch, expect it to fail after switching to DOT-5. Some say the Harley brake light switch is compatible and will work. Others convert from a hydraulic switch to a pedal actuated switch.


    Once a rubber part has been used with one fluid, it will not work well with other types. That means new or rebuilt wheel cylinders and rebuilt calipers. Your best, longest life will come from replacing all the contaminated rubber bits and thoroughly flushing the old metal lines.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

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    Re: Flushing Hydrulic Fluid

    Quote Originally Posted by frankfast View Post
    I am told that DOT5 will mix with DOT3 and 4...
    Not only will it not mix, but the DOT3/4 will suck the purple color out of the DOT5. When I first converted using the brute force / crude method mentioned above the DOT5 turned colorless. Now, after many years and subsequent flushes/replacements it retains its purple color - suggesting that either most of the old DOT3/4 is gone or that what is hiding somewhere is saturated with the purple dye.

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    Re: Flushing Hydrulic Fluid

    I stand corrected. I read that DOT5.1 will mix, not DOT5.

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    Re: Flushing Hydrulic Fluid

    Right, DOT 5 won't mix. However, it is compatible, the two can co-exist peacefully. In my experience, it seems that the non-DOT 5 absorbs not only the purple dye, but also any moisture and wear particles, etc. such that it eventually turns into a black goo. Unsightly, but doesn't seem to present any problem except perhaps (if it settles inside a caliper) boiling easier than DOT 5.

    DOT 5.1 is not silicone, and will still eat paint, absorb moisture, degrade over time and so on. Better than DOT 4 (higher BP and all), but not as good as DOT 5 (for our old cars, IMO).

    I agree that "best practice" is to replace all the seals, soft lines and so on. Preferably using the SS/teflon soft lines instead of rubber. (Good time to think about replacing those antique hard lines too.)

    However, I'm not convinced that it is always essential to do so. For example, I converted a 1980 Chevy to DOT 5 in 1988 by just bleeding through at all 4 corners until clear purple fluid came out. One caliper was replaced (it was leaking before), everything else remained original. I never touched the brake hydraulics again, they were still working perfectly when the car was junked in 2005 (with roughly 250,000 miles on it).

    If you do want to flush the lines, you might consider using brake cleaner rather than rubbing alcohol. It's a much better solvent, and unlike rubbing alcohol, doesn't contain water. Personally, I wouldn't use shop air either, as it is often contaminated with water and/or oil. Or at least use a filter on it.
    Randall
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    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Re: Flushing Hydrulic Fluid

    I followed Randall's advice and converted to Silicon.
    My lines had been removed from the car during the restoration (still going on), emptied but not flushed.
    There was a bit of the old Dot 4 fluid pushed out by the silicon but not alot and my brakes work perfectly.
    The silicon seemed to be harder to bleed and was alot more expensive in New Zealand but I'm sure it's worth it as the old fluid ended up corroding all my hydraulics.

    Jim and the stationary TR4

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    Re: Flushing Hydrulic Fluid

    Does anyone know how much fluid is need to refill the entire system? Is a one quart bottle enough to complete the task?
    Life is an adventure. It's not the destination we reach that's most rewarding, it's the journey we travel... every day is a gift!

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    Re: Flushing Hydrulic Fluid

    The capacity of the system is less than one quart though, depending on what you are doing, you may use the rest of the quart for flushing.

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    Re: Flushing Hydrulic Fluid

    FYI - denatured alcohol and isopropyl alcohol are two different things.

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    Re: Flushing Hydrulic Fluid

    Where can one find isopropyl alcohol? They don't sell it at an auto parts store.

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    Great Pumpkin TR3driver's Avatar
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    Re: Flushing Hydrulic Fluid

    Isopropyl is commonly available at drug stores or perhaps even the grocery store. Just ask for "Rubbing alcohol".

    I actually used Methanol (yet another kind, aka "wood alcohol", aka methyl alcohol) back when, as it is supposedly compatible with rubber brake lines (the other two aren't). Harder to find, but my drug store had it behind the counter. It's much more poisonous than the other two, so they don't put it out on the shelf.
    Randall
    56 TR3 TS13571L once and future daily driver
    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Re: Flushing Hydrulic Fluid

    Read the label on the rubbing alcohol. There are still some brands that are denatured ethanol. Both the ethanol and isopropyl rubbing alcohol are typically a certain percentage water. Isopropyl is a better solvent. If you choose to use rubbing alcohol, be sure to blow the lines dry.

    Electronic supply firms sell isopropyl without water.

    Also look for it by its other name, isopropanol.
    Doug L.
    '64 Morris Mini Cooper-S 1275
    '67 Triumph GT6 Mk1

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    Re: Flushing Hydrulic Fluid

    Quote Originally Posted by TR3driver View Post
    Right, DOT 5 won't mix. However, it is compatible, the two can co-exist peacefully.
    Right. I don't know how the very popular idea that DOT5 and DOT3/4 aren't compatible and will destroy the world if mixed came about. US DOT regulations actually require them to be compatible, and have stringent compatibility tests that must be performed on the fluids before they can be sold. Extensive testing in Europe in the 70's showed a 50/50 mix of the two will work without issue for many miles of driving. The main problem with mixing them is that you don't know exactly what percentage you have and so you don't know your boiling point. Not usually a big issue on street cars. The Australian army did some testing in the 80's looking at what happens when you convert from DOT3 to DOT5 by flushing. What they found is that you will always have a non-insignificant amount of left over DOT3 in the system, mostly in the bottom of the calipers (because the bleeder valves are meant to bleed air, not fluid) and it is very hard to determine how much exactly is left, so they abandoned the practice.
    Jeremy

    '60 TR3A, '65 Spit Mk2, '73 GT6 Mk3

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    Re: Flushing Hydrulic Fluid

    After the previous threads we came to a limited conclusion that Silicone is likely better in dry climates, where there is little moisture to settle to the bottom of calipers and cause rust damage. Glycol fluid is likely better for wet climates, as it absorbs the moisture, preventing it from settling to the bottom and causing rust. Of course you would have to change the glycol fluid before is saturates.
    John

    1955 TR2

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