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Thread: Brakes for the Impala
          
   
   

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  1. #1
    Dave Severson is offline CHR Member/Contributor Visit my Photo Gallery
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    Brakes for the Impala

     



    Ok, so most of the early cars with drum brakes all around ran a bigger brake line to the rear of the car, I always figured this was because the old wheel cylinders/drum brakes required more volume of fluid? Anyway, converting the '66 over to 4 wheel discs, there should be no reason to have the larger line to the rear, is there? Thinking it will be fine to have all the lines as 3/16" withe the brake hoses as -3 Stainless?
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    I'm thinkin' that once all the lines are filled with fluid, the rears will not know that they are not on the front and vice versa. You run the same size left to right and that works ok, right?
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    Quote Originally Posted by techinspector1 View Post
    I'm thinkin' that once all the lines are filled with fluid, the rears will not know that they are not on the front and vice versa. You run the same size left to right and that works ok, right?
    .
    That's what I thought too, just wanted to check and see if I was overlooking some reason for the larger line to the rear.....
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    I always figured it was a flow situation too. On most pick ups they run a 1/4" line to the rear and at the flexible hose it goes to a T and into 3/16" most times. I think it would be fine leaving the larger line.

    .
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    Interesting question.

    I just did about 20 minutes of research and this is what I found:

    Hydraulic theory: brake fluid does compress but so little as to be insignificant.

    Tubing does expand when pressure is applied. The bigger the tubing, the more it will expand. Enough to be noticeable? Opinions vary.

    Bleeding: might be easier with smaller tubing.

    Volume: we're not flowing fuel. Brakes function with very little flow. If ya don't believe me, do the math. It's about pressure. Larger tubing will flow at lower resistance. Significant amount? Doubtful.

    The more I think on it, the more I expect the answer is going to be related to manufacturing costs, not performance.
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    I'm wondering if it was a formal calculation tied in with wanting to load the front brakes slightly ahead of the rears? Just seems to me that for a given pedal movement/pressure with 3/16" line to the fronts and 1/4" line to the rears, the pressure is going to increase just a tiny bit earlier in the fronts than the rears, just due to the volume difference, but then will equalize. I'm not sure of that, but it kind of makes sense to me.
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  7. #7
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    Aggressive driving (or using a lot of brake when towing) generates heat. Brake pads can reach extremely high temperatures (OEM brake pads operate best in the 100F to 650F range while racing and high performance pads need to be at 500 F plus to become efficient just about where stock, OEM pads begin to fade.) Some of this heat is passed onto the brake fluid through the brake calipers, which can raise the fluid temperature to well over 300F. Fortunately, the boiling point for brake fluid is significantly higher than water. DOT 3 boiling point is 401 F dry and 284 F wet. DOT 4 boiling point is 446 F dry and 311 F wet. DOT 5 boiling point is 500 F dry and 356 F wet. (Wet boiling point defined as 3.7% water by volume.)

    If the brake fluid is repeatedly heated past its boiling point, some of the fluid vaporizes and creates air bubbles within the brake lines. This can lead to a bad and potentially unsafe situation where the brake lines are carrying air instead of brake fluid. Air can be compressed which is why it must be bled out of the systems so that the fluid can properly move the pistons inside the brake caliper and properly operate the brakes. High temperatures also promote condensation (from vapor) and water develops in the lines (ever seen rusty brake fluid?) and this is a huge problem as shown in the above figure wet boiling point is much lower.

    Fluids dont compress much (in liquids, there is no gap between particles, so only way you can compress fluid is by reducing the gap at a molecular level, which is effectively changing the state of that material, which involves a lot of energy but as mentioned, brake lines do flex.

    The advantage to larger lines is greater volume of fluid to dissipate the heat and keep the system well below the boiling point. With DOT 5 fluids, youd be hard pressed to generate enough heat to boil fluid in a properly operating system regardless of the line size.
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  8. #8
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    Quote Originally Posted by rspears View Post
    I'm wondering if it was a formal calculation tied in with wanting to load the front brakes slightly ahead of the rears? Just seems to me that for a given pedal movement/pressure with 3/16" line to the fronts and 1/4" line to the rears, the pressure is going to increase just a tiny bit earlier in the fronts than the rears, just due to the volume difference, but then will equalize. I'm not sure of that, but it kind of makes sense to me.
    I agree, but conditionally:

    It's not volume (do the math) it's that the larger tubing stretches out more, which results in a pressure differential.
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    Also, in reference to the water vapor comment:

    Brake fluid is hydrophilic. Which means that it will suck water right out of the air. That's why your brake lines get rusty
    .
    Education is expensive. Keep that in mind, and you'll never be terribly upset when a project goes awry.
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    I believe I'll just use 3/16" line throughout the system, of course making sure I get a master cylinder designed for disc/disc. Thanks everyone for the input! I've always used same size line on race cars and drag cars, I can't see a reason that it won't work just fine on the street. Probably put in a proportioning valve in case tuning brake pressure front to rear is necessary. Appreciate the help!
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  11. #11
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    Will this be a straight mechanical system or will it be assisted in some way?
    .
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  12. #12
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    Quote Originally Posted by techinspector1 View Post
    Will this be a straight mechanical system or will it be assisted in some way?
    .
    I'll be using an 8" dual diaphragm booster. Just a cruizer, I've got to have all the "easy things" for Jackie, ya know? lol
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