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Mixing DOT3 and synthetic brake fluid

Discussion in 'The Hokey Ass Message Board' started by Redbows35panel, Oct 15, 2012.

  1. Redbows35panel
    Joined: Dec 29, 2009
    Posts: 165

    Redbows35panel
    Member

    I've found lots of articles that say you should not mix brake fluids. But I have found nothing about what happens if you do. I purchased a restored car, and as I was checking it out after the purchase. I found the brake fluid was low, so I added about half the cylinder of DOT3. Several weeks later while talking to the guy that restored the car he mentioned that it had synthetic fluid in it. That's been three months ago and I still have not noticed any change in the brakes. What should I watch for???????????? I'm thinking of just replacing the master and wheel cylinders and blowing out the lines. Then just using DOT3. Will that solve in potential problem:confused:
     
  2. I didn't even know there was such a thing.

    Probably just a flush would do it, fluid gets old and gets dirty and still works about the same, a few traces probably won't hurt anything.
     
  3. It makes jelly out of the fluid with dot 5.
    It gums up piston return and causes blockages.
    Flushing rarely gets rid of the problem.
     
  4. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865

    unkledaddy
    Member

    Rest easy.

    All DOT3 is full-synthetic...........as well as Dot4 and DOT5.1. The one you don't want to
    mix with the others is DOT5 as it's silicone.

    Some manufacturers like to enhance the word "SYNTHETIC" on the container but in actuality they're all full-synthetic except DOT5.
     

  5. bauschracing
    Joined: Mar 31, 2011
    Posts: 64

    bauschracing
    Member

    The brake fluid will jell up. The military has been using silicone brake fluid for years. When it first came out, we had all type of trouble with the brake fluid jelling up. The DOT and silicon will not mix and function properly. There must be a leak in the brake system since the silicon fluid does not evaporate. I would completly flush the entire system and get the mix out before trouble starts. Replace/repair the wheel cylds, master cyld, etc. and use either DOT or silicon. There are advantages to the silicon, you can spill it all over the paint and it will not lift the paint. Moisture is not a concern either. I have seen the silicon fluid for sale in corvette parts ads. Just get the mix out right now even if you leave the system empty.
    Mike
     
  6. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865

    unkledaddy
    Member

    From what the OP said it sounds like he has only synthetic brake fluid in his system,
    be it DOT 3, 4 or 5.1. The previous owner used synthetic fluid and the OP added
    DOT 3 which is synthetic.

    I certainly wouldn't do any more than fill the MC and keep an eye on it, as well as
    looking for a leak somewhere if the MC doesn't stay full.

    Contact the previous owner/restorer and I'll bet he'll tell you he used DOT3.
     
    Last edited: Oct 15, 2012
  7. rustang
    Joined: Sep 10, 2009
    Posts: 710

    rustang
    Member

    You will be fine....synthetic is not silicone

    Just NEVER mix DOT 5 with any of the others! I hate that they came out with DOT 5.1, as this would be a real easy thing to mix up with DOT 5.... then you have problems.
    Tom
     
  8. Normal Norman
    Joined: Aug 9, 2006
    Posts: 510

    Normal Norman
    Member
    from Goshen IN.

    I would flush the system and start over with whatever fluid you prefer. What hasen't been mentioned here is you need to use alcohol to flush the system. Normal Norman
     
  9. Weasel
    Joined: Dec 30, 2007
    Posts: 6,698

    Weasel
    Member

    What also has not been mentioned here is that DOT5 silicone will slowly erode normal rubber type seals and result in sudden and total brake failure - ask me how I know. If you feel that there is a compelling reason to use DOT 5 silicone then you must replace every single rubber component in your brake system with neoprene type seals which are DOT 5 compatible. I can see zero good reason to use DOT 5....
     
  10. manyolcars
    Joined: Mar 30, 2001
    Posts: 9,176

    manyolcars

    I have never hear of DOT 5.1 until recently and have never seen any for sale.
    Who sells it?

    I have mixed DOT3 and DOT5 in a cup on the workbench and watched it for weeks.

    nothing happened
     
    Last edited: Oct 15, 2012
  11. manyolcars
    Joined: Mar 30, 2001
    Posts: 9,176

    manyolcars

    I have been using DOT 5 on many ol cars for about 25 years because the rubber parts do not deteriorate. I've never had a 'sudden and total brake failure '
     
  12. Tommy's Cycle
    Joined: Mar 22, 2006
    Posts: 766

    Tommy's Cycle
    Member
    from So Cal

    5.1 is basically just a high performance DOT 4 and has a slightly higher boiling point. I've seen it at Autozone & O'Reilly's as well as most speed shops. DOT 3, 4 & 5.1 are all glycol based fluids and are hygroscopic, ( absorbs water). Mixing with a silicone will eventually cause any moisture aborbed by the Glycol base fluids to puddle and gel. As stated above "Synthetics" & "Silicones" are not the same.
     
  13. ddphillips
    Joined: Jan 27, 2012
    Posts: 13

    ddphillips
    Member
    from Cincinnati

    Does the car have disc brakes? If so, the fluid level will go down as the brakes wear.
     
  14. Redbows35panel
    Joined: Dec 29, 2009
    Posts: 165

    Redbows35panel
    Member

    It's not synthetic fluid that was in the system, but silicon. Being the DA that I am I said the wrong thing. So it appears from the comments, that the best solution will be to replace the MC and WCs. Then do I flush the lines with what? And NO it does not have disc brakes, but now might be a good time to install them. This ole Pontiac is becoming more of a Street Rod every day. Just brought it home from Cruisin the Coast. Over 6000 registered cars this year. Don't think I saw a shabby one in the bunch.
     

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