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Does brake fluid boil?

Discussion in 'The Hokey Ass Message Board' started by FatfenderJ, Oct 25, 2010.

  1. FatfenderJ
    Joined: Oct 6, 2004
    Posts: 154


    Periodically I have to add brake fluid. I always figured there was a seeping wheel cylinder but have never been able to pin point it. So anyway this August I was up in Spokane and the pedal felt funny so I figured it was time to add fluid. It was almost dry and while I had the floor board up it occurred to me that the master cylinder is right next to one of the exhaust pipes. It's a fruit jar type master and I think it might be boiling over. Is this possible or likely?
  2. BulldawgMusclecars
    Joined: Jul 15, 2010
    Posts: 508


    Yes. I'd reroute the exhaust if possible, or make a heat shield.
  3. Any liquid will boil if you get it hot enough.
  4. swblandy
    Joined: Feb 25, 2010
    Posts: 22

    from scotland

    Yes brake fluid can boil ,either reroute the exhaust or fit a shield and possibly use dot 5 fluid
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  5. Here is the boiling point of brake fluid and also the effect of moisture on the boiling point

    Attached Files:

  6. skidsteer
    Joined: Mar 19, 2007
    Posts: 1,251


    Yes, and to avoid just that, I put a 'Y' pipe under my engine, ran a single exhaust pipe down the passenger side of the frame, then split it before the rear end, to have dual exhaust pipes out the back.
  7. junk yard kid
    Joined: Nov 11, 2007
    Posts: 2,720

    junk yard kid

    Wow, just a little moister really reduces the boiling point.
  8. V8 Bob
    Joined: Feb 6, 2007
    Posts: 2,627

    V8 Bob

    Brake fluid does boil, but at very high temps near 400 F, depending on moisture content. When it boils, you will loose most/all braking, but not the fluid! You have a leak somewhere in your system.
    Joined: Jun 3, 2005
    Posts: 8,042


    Good place to bring this up, I read that silicone fluids boil at very low temps in high altitudes. I was shockd and didn't believe it but they tested some fluids and sure enough, altitude has an effect on silicone's boiling point. Just something to remember/ponder.
  10. joep4567
    Joined: Jul 16, 2010
    Posts: 20


    i'm not sure but doesn't altitude effect the boiling point of all liquids?
  11. Von Rigg Fink
    Joined: Jun 11, 2007
    Posts: 13,428

    Von Rigg Fink
    from Garage

    I believe that to be true.
    but its just a gut feeling

    but silicone may be the reason its boiling at a lower temp?
    wonder why its in there to begin with?
  12. squirrel
    Joined: Sep 23, 2004
    Posts: 46,117


    somthing about physics....less pressure holding the molecules together at lower elevation, means it'll boil at a lower temperature.

    Even if it boils, where's the vapor going to go? Should have a sealed system, there's a bellows under the cap on the m/c, right?

    Might want to look for leaks, check all fittings for tightness, and maybe rebuild some cylinders or get new ones.
  13. GassersGarage
    Joined: Jul 1, 2007
    Posts: 4,729


    As a matter of fact, it does. In the old days, like real old days, expeditions to the arctic used the boiling point of water to determine altitude.
  14. falconsprint63
    Joined: May 17, 2007
    Posts: 2,359

    from Mayberry

    yes and when it does the pedal gets real soft. had that problem with a hydrolic clutch once.
  15. GassersGarage
    Joined: Jul 1, 2007
    Posts: 4,729


    I had the opposite effect, the pedal got hard. I was running disc brakes on front and they would drag until I bled off the pressure.
  16. aaggie
    Joined: Nov 21, 2009
    Posts: 2,529


    Probably having a senior moment but I think the "fruit jar" master cylinders had a vented cap.

    DOT 4 and up brake fluid was developed to prevent brake fluid boiling from the calipers getting hot on race cars. Disks would get red hot and pedal would go to floor.

    Dot 4 and 5 fluid is much more likly to absorb water from the air and should/must be used with stainless brake parts to keep rust out of the system.

    For hot rod use Dot 3 fluid and a heat shield should fix his problem.
  17. falconsprint63
    Joined: May 17, 2007
    Posts: 2,359

    from Mayberry

    funny how things manifest. mine was a small manfold leak spitting hot air onto the slave cylinder. got so hot I just lot pedal and the ability to engage the cluch. had to dump a couple of bottles of water from the cooler on it to cool it down enough get it the mile or so back to the condo in gatlenberg traffic. fixed leaad and added insulation to the exhaust system andhave had no further problems.

  18. pull the wheels, pull back the boots, pull back the boot on the master cylinder,the fluid is leaking somewhere. power booster can leak and suck it into the engine. if master is facing backwards [pedal linkage side facing forward] it tends to make them leak prematurely. stuff a tissue in the boot and drive it then check the tissue for fluid.
  19. Say whaaat?
  20. aerorocket
    Joined: Oct 25, 2007
    Posts: 488

    from N.E. P.A.

    Less pressure does equate to lower boiling point however lower pressure occurs at higher not lower altitude.
  21. Robert
    Joined: Mar 20, 2001
    Posts: 94


    Wagner SevereDuty

    Wagner SevereDuty brake fluid is not silicon-based and can be mixed with DOT 3 and DOT 4 fluids. This makes it an excellent choice for fleets, such as taxi cabs and emergency vehicles, and for racing applications. Wagner SevereDuty has a dry boiling point higher than 500° F (260° C).
  22. squirrel
    Joined: Sep 23, 2004
    Posts: 46,117


    uhhh...higher elevation. Sorry for the brain fart.
  23. Triggerman
    Joined: Nov 18, 2006
    Posts: 578

    from NorCal

    Yes it does and I speak from experience. I used DOT 5 in an OT Corvette and didn't read the label until the first bottle (BIG bottle) was poured in. The manufacturer recommended heating the fluid in a pot to "remove the entrained air from the brake fluid). Long story short I lost the pedal going over the Sierra Mtns on the way to Reno. High compression and a four gear got me down the other side and as the elevation came down my brake pedal came back.

    Moral of the story is unless you are super concerned about moisture contamination or paint damage from the brake fluid stay away from DOT 5.
  24. Clik
    Joined: Jul 1, 2009
    Posts: 1,841


    My first experience with boiling points was when I was a little kid visiting granny up in the mountains. She always canned with a pressure cooker while my mom who canned at near sea level did not. Granny explained that the water would boil off before reaching the temp required to sterilize due to the altitude.
  25. tgabbe1934
    Joined: May 20, 2009
    Posts: 64

    from smithtown

    I think it Does, maybe you can use header wrap on the pipe. It might help.
  26. Clik
    Joined: Jul 1, 2009
    Posts: 1,841



    I wonder if your experience was due to low temps at high altitude. DOT 5 is hydrophobic. It doesn't absorb water. Water will pool and freeze in cold weather where it would be mixed in DOT 3 and not freeze. DOT 5 has a very high boiling point and I can't imagine altitude lowering the boiling point anywhere close enough to boil.

    If the car wasn't thoroughly flushed after using DOT 3 and then adding DOT 5 it would probably have some water in it which could pool and freeze.
  27. 53sled
    Joined: Jul 5, 2005
    Posts: 5,819

    from KCMO

    Everything boils and freezes. But I think you have another problem. I've had them leak into the booster before.
  28. Bosco1956
    Joined: Sep 21, 2008
    Posts: 545

    from Jokelahoma

    Brake fluid should be flushed on a regular basis. On my cars that aren't driven much I try to change it every couple of years. On my daily driver I flush the brake system everytime I do any brake work.
  29. Just for fun and FYI. I always like to have a comparison of something I understand when dealing with physics. Think of the atmosphere (air) as water, after all it is considered a fluid. What happens to the pressure when you dive deep in the ocean? It increases. Why? Because of the weight of the water. The same principle applies to the atmosphere- the air is piled higher on top of us at lower elevations.
  30. Retro Jim
    Joined: May 27, 2007
    Posts: 3,860

    Retro Jim

    Yes brake fluid will boil if hot enough . The heat will affect your brake fluid and if it given enough time to get hot enough , it just might brake down enough till it won't work like fresh brake fluid will do . I don't know all the chemistry of what happens to brake fluid when it reaches a certain temperature but I don't want to find out one day doing 60 mph and have to stop suddenly !
    I alway try to keep all liquid carrying lines away from all heat that is generated from the engine and the exhaust ! To me that is just common since for pure safety !

    Retro Jim

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