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Thread: mixed dot 3 and 5 brake fluid

  1. #21
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    Rustystud, are you saying that if you change from one fluid to the other, you should also change all your seals? I'm curious now what is going on with the seals? If you go from 5 to 3 is there a difference from 3 to 5? I can see that after a change from one Dot to the other there would always be the question if all of the old was out, which raises the question what happens if there is a small amount remaining? because unless some kind of solvent was used or you replaced all old seals with new seals the old seals would still be wet with the old fluid. Is that where you are coming from? I've read many posts where guys have converted to dot 3, but I haven't read where they replaced all seals. I haven't read where they had problems after the change.
    The fluid in my MC was not separated. I sucked fluid from the bottom and the top of the MC reservoir and it looked the same. The fact that the MC access door was labeled Dot3 and there was a partial bottle of Dot 4 in the tool box means a change was made. How well that was done I don't know. Right now if I pump the pedal 3 times I have brakes if I pump again I have more pedal. If I let off the brake pedal I have no pedal until I pump again. Would this be an air in MC problem? or a MC failure. I see no exterior leaks. Thanks

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    I suggest you do a complete brake system overhaul.

    Use which ever fluid you choose.

    Adjust all shoes.

    Rebuild/seal everything.

    Change all rubber hoses.

    Inspect and replace any steel lines that show any rust.

    Anything less and you're gambling(guessing).

    PS, And while you have it torn down, inspect and relube all wheelbearings and replace any seals that are leaking too.
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    I suggest you follow Dogheads advice here. The reason is the fluids are completely different base materials and they "do not mix !" . They actually will damage the seals from either system. DOT 3 to DOT 5 or DOT 5 to DOT 3. Once the seals absorb the fluid they never will be free of it. That is why you hear stories of guys who did convert and a year or so later are complaining of damaged and falling apart seals. Read some of the articles Frank posted. There is some excellent information there.
    I've converted over 10 vehicles to run DOT 5. Had excellent results, but I completely rebuilt every system. Even the master cylinders where rebuilt with new seals. After that I never had a problem with any of them. In fact I posted a few years ago
    information on my CUCV and it's DOT 5 brake system. The calipers where like brand new inside after 20 years running silicone brake fluid. I still replaced the seals just for safety reasons, even thought they where like new.

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    The part about the seals absorbing the original fluid and the fact that getting 100% of the old out is the key point in my mind. I would like to hear from guys who had a failure afterward....I've read about many changing to dot 3, but have not read about the long term results. The seal failure may be what is going on in my truck.
    Last edited by 2deuce; 01-10-2019 at 21:04.

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    With single circuit brakes, there is just no way to cut corners here. If you have a failure, you'll lose everything. It's a dangerous experiment to see if you can just push a different fluid down the lines and "see what happens." I just don't see how you can purge 100% of the old fluid without disassembling the master cylinder and wheel cylinders. There will always be a small amount of trapped fluid, contaminating the new fluid.

    Once you decide which type of fluid you're going to use, I'd pump a bunch of it through the entire system and drain it via the wheel cylinders. This will clear the metal lines. Do this BEFORE installing new master and wheel cylinders, that way you won't contaminate the new equipment with the mixed fluid. *Then*, remove and rebuild/replace the master and wheel cylinders, add fluid and bleed the system.
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    I agree that in changing from one dot to another, trace amounts if not more, will remain in the system. The truck I have does have dual circuit brakes. Probably the question that is the most concern is how much of these trace amounts matter. Especially as brought up earlier about the old fluid already absorbed by the seals. Obviously all new seals is the best option and how can anyone advise against that. My truck was changed over with cost of Dot 5 being the motivator to go to Dot 3. There always are too sides to any situation and I would like to hear from others and what they experienced with Dot 5 to 3 transfusions years after. I've read many posts where it was done and none where they had problems that I can remember. Before I go to all the time and expense of changing out all the seals, rebuilding the MC and 2 air packs I think I might try bleeding the system and see what comes of that. If I have a leak or poor function then replace everything.

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    Greetings,

    Not an MV but a motorcycle.

    When DOT 3 / 4 is mixed with DOT 5 the result is a yellow thick sludge that resembles Gorilla Glue. Bad news!

    I rebuilt the rear brake caliper and also replaced the master cylinder. The brake lines, both steel and rubber were blown through several times with brake cleaner and compressed air. I also rebuilt the front master cylinder and brake caliper with the same blowing out of the lines. Then I used DOT 5 fluid.

    No issues in over one year.

    Why did I do this? The rear brake would come on but did not want to release. Not much fun on a bike. Also any slight leak of DOT 3 / 4 would corrode anything that it could.

    I could have gone with DOT 3 / 4 but since I already had DOT 5 for my Deuce I went with that.

    For identification purposes I sucked out about 1cc with a syringe and then added another 1 cc of water, shook it up well. The DOT 3 / 4 mixed with water and would not separate after several hours. The DOT 5 separated into fluid and water in less than an hour.

    Hope this helps.

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    I firmly believe DOT 5 is superior to DOT 3 or 4. I've used DOT 5 in my deuces, motorcycles and some cars, and have never had a problem. It is not hard to bleed the brakes on a deuce, just follow the instructions. DOT 5 is only prohibitively expensive if you go to the auto parts store. It is readily available surplus. I buy it by the gallon can. Flush the brake system, rebuild the brakes, switch to DOT 5 and forget it. Brakes are the most important part of the vehicle.
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    Gary, where do you get Dot 5?

  14. #30
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    Quote Originally Posted by 2deuce View Post
    Gary, where do you get Dot 5?
    The last time I bought it on Ebay. I bought 6 gallons of military surplus. There is almost always bulk military surplus DOT 5 on Ebay or MV parts suppliers. Don't buy DOT 5.1 as it is totally different from DOT 5. The time before that I bought two gallons at the Georgia Rally.
    Gary
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    1969 Kaiser Jeep M35A2 W/W (Sold)
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    1968 Johnson M105A2 Trailer
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