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Old 02-23-2007, 08:45 PM
brian berwick's Avatar
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brake line size

Iknow its not a hotrod but can anyone tell me what size brake line that runs to the rear breaks on a 1988 chrysler fifth ave its a caniadian car

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Old 03-01-2007, 07:33 PM
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Line diameter makes no difference. Just get line w/ end fittings that fit your master and slave cylinders.
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Old 03-01-2007, 08:01 PM
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Pretty sure 3/16" tubing/flare.
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Old 03-02-2007, 04:32 AM
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prolly 3/16". line size does affect pascals law, but, even if you used 1/4" brake line, it'd work ok, it's not going to make a big diffrence. ends prolly are not 1/8" so you'll likely need some adaptors, to adapt it to what you need.
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Old 03-02-2007, 11:48 PM
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Hmmm . . . . I'm interested in what you mean by 'line size affects Pascal's law'. All that principle states is that a pressure applied to a confined incompressible fluid is transmitted equally to all contacted surfaces. It is not concerned with the shape of the containment which could be the passenger compartment of a 747 jet liner or a 3/16" brake line. The only hydraulic parameter that is affected by line size is friction pressure loss due to moving fluid. Since there is so little fluid displacement in a brake system, line size is not a factor. Any commercially available line will work fine in an automotive application. The biggest problem in hotrodding is finding adapters for the crazy end connectors that the OEM folks come up with.
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Old 03-03-2007, 06:12 PM
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Quote:
Originally Posted by willys36@aol.com
Hmmm . . . . I'm interested in what you mean by 'line size affects Pascal's law'. All that principle states is that a pressure applied to a confined incompressible fluid is transmitted equally to all contacted surfaces. It is not concerned with the shape of the containment which could be the passenger compartment of a 747 jet liner or a 3/16" brake line. The only hydraulic parameter that is affected by line size is friction pressure loss due to moving fluid. Since there is so little fluid displacement in a brake system, line size is not a factor. Any commercially available line will work fine in an automotive application. The biggest problem in hotrodding is finding adapters for the crazy end connectors that the OEM folks come up with.
kno what you mean with the crazy connections, 1 end needs a double flare, 1 end needs a ISO/bubble flare WTH, now you need a $300 flaring tool, just to make a $10 brake line work??? GM is noted for doing this. line size can affect pascals law, creates more PSI when you do the math. it was confusing for me to learn it, because it's backwards of what I normally deal with ( air preassure ). but the bigger the area, the more preassure there will be applied on all surfaces in the hydraulic system.
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Old 03-03-2007, 06:38 PM
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Not more pressure as the line gets bigger. Pressure is a constant regardless of the surface area, that is what pascal says, regardless of whether you are dealing w/ compressed gas or incompressible liquid. "Pounds" per "square inch" is the constant so a 0.0001sqin surface sees the same exact pressure as a 1000sqin surface. Now the force a given area sees is a different thing. @ 100psig, they both see that pressure but the smaller surface will see a force of 0.0001 x 100 = .01pounds-force. No big deal, you can hold that back with a thumb. The bigger surface will see 1000 x 100 = 100,000 pounds-force. That will take both thumbs and probably a little help form your brother-in-law to resist. That is the only reason I can figure why Detroit uses different size brake lines. If you consider the end of a fitting is a piston (the fluid does so you might as well too), the the larger the diameter of the line, the more force there is trying to eject the fitting out of the threaded hole.
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Old 03-03-2007, 06:46 PM
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At 1800 psi, the amount of pressure swells the hard steel line. There is much less swelling in a 3/16" line than a bigger one. If you don't believe it, install a 2' section of 1/2" tubing inline to your rear brakes and tell me they don't feel different.

Willys is right. Its the same principle that makes car hoists work. 100 psi generated from a master cylinder with 10 square inches of swept area will equal 1000 pounds of force in a slave cylinder of 100 square inches.

The fluid pressure seen inside the tubing is still 100 psi, but the physical forces on the tubing wall increase as the surface area of the tube increases. That has a notable effect on brake effecacy.

larger diameter tubing has lower psi ratings for the same wall thickness for that reason.

But its a moot point... 3/16" will do. If you're worried about hooking up to stock hardware, ask the parts store to pull brake hoses for that car and match them up. 3/16" line comes standard with 3/8" fittings (I think) but if you want you can buy tubing nuts that fit the tubing on the inside and fit any size fitting on the outside. Cut the old flare off, put the nut on, reflare.

Do yourself a favor. We've all done these things and it seems to work fine, but avoid two things and you'll sleep better at night: 1) don't use brass fittings. The 1500 psi that your brake lines can consistently see out-does brass's rating by tenfold. I've seen brass fittings just push right out... literally ripped the threads straight out of the union. 2) you're going to get frustrated with the double flare tool. Don't skimp and settle for a single flare. Singles won't leak so you'll be fooled into thinking they're OK but they're not. Every time you hit the brakes the whole joint flexes a tad. Single flares are very prone to cracking. Once that happens the joint can fail under a panic stop and then you really panic. Take the time to square the cut, deburr inside and out, carefully double flare, have peace of mind when the loved ones are in the car and a milk truck pulls out in front of you.
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Old 03-03-2007, 07:32 PM
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3/16" comes with 1/8" fittings, 1/4" have 3/8", unless your getting an OEM line. Brass fittings are only DOT legal on the master Cylinder, for bushing/ reducer's, but brass is legal, for coupling 2 lines togther.
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