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  • Brakes: Hydro-vac

    Thirty five years ago I completed the restoration on my 1955 Speedster. The brakes were rebuilt but not the Hydro-vac. The car was only twenty years old and the HV was still working good. If it ain't broke don't fix it! I knew that the piper would at some point have to be paid, but it just wasn't his time.

    Silicone brake fluid was recent arrival on the automotive scene and seemed to be the way to go, so I used it. There were few cautions regarding it's use and no negative information regarding a HV equipped car. It was years later that I began to hear about the real chance that the fluid will be sucked into the engine by vacuum from the HV, not good! The car has been used sparingly since the restoration, but the brakes have been a non-issue for the last thirty five years, until the other day. I guess that this can serve as an advertisement for SF (Dot 5). The two cars that I own that I used Dot 5 in performed flawlessly for thirty five years, I wish that I could say that about my other cars in which I used Dot 3. It had been over a year since the Speedster had been started and run, but after an oil change and some prep it started and ran good. However there was no brake peddle and pumping did no good. Thinking that there was a fluid leak and the piper might be ready to collect his due, I checked the master cylinder, but it was still full of fluid. I haven't yet checked to see if the piston in the MC is stuck, but it is difficult to tell if there is any resistance at the peddle. Ideas anyone?

    I know that it is strongly recommended that all the rubber parts be changed when going from Dot 3 to Dot 5, but has anyone been successful flushing their hydraulic system, with alcohol, without changing the rubber parts before making the switch? What about going the other way, switching from Dot 5 to Dot 3/4. Unlikely that many have gone in this direction but in a HV equipped car I would now consider it.

  • #2
    It wasn't the DOT5 which caused a problem with the hydrovac.

    Has anyone seen any reliable data that sucking silicone fluid into the intake is anyhow more damaging to the engine than the DOT3/4? That's really nasty stuff.

    jack vines
    PackardV8

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    • #3
      Originally posted by PackardV8 View Post
      It wasn't the DOT5 which caused a problem with the hydrovac.

      Has anyone seen any reliable data that sucking silicone fluid into the intake is anyhow more damaging to the engine than the DOT3/4? That's really nasty stuff.

      jack vines


      How about this...

      Click image for larger version

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      64 GT Hawk (K7)
      1970 Avanti (R3)

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      • #4
        I think it would be premature to consider changing to DOT 3 before you even know for sure what is wrong with your brakes. Hydrovacs go bad on all types of brake fluid, and it probably had nothing to do with the DOT 5 fluid. If you're convinced that it's the Hydrovac, then why not make up a short jumper brake line and cut the Hydrovac out of the circuit? You should then have brakes again, albeit with a hard pedal.

        I have never heard of anyone successfully switching between DOT 3 and DOT 5 without changing the brakes hoses and replacing the rubber parts in the M/C and wheel cylinders -- but I've heard of several people who tried and had problems.
        Skip Lackie

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        • #5
          I've used DOT 5/silicone fluid in all my Studes-disc and drum and have had no problems. At the suggestion of my inspection mechanic I add a few drops of DOT 3 fluid to the stoplight switch before installation. This adds to the life of the switch. As for booster rebuilds I've used "Booster Dewey" for my MasterVacs and Ed Strain for my HydroVac boosters. Both advertise in Hemmings and are available by phone if there are questions.
          Rob in PA.

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          • #6
            Many years ago I did a complete brake overhaul on my 1949 Chevy 1 1/2 ton truck. It doesn't have HydroVac, but I did use DOT 5 silicone and everything works fine. After the truck set unused for a year the pedal did go to the floor or close to it, but soon pumped back up and worked fine again.

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