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Discussion Starter · #1 ·
HELP!!! I need to know what size brake lines to run the back drums on my T-bucket. I ran 3/16 to the front discs and need to know if bigger is better or same size for the drums. Is the volume and pressure thing going to apply here? Do I need more volume or more pressure for the drums. I have an adjustable proportioning valve I can put in on the fronts. Is this a good idea??

COME ON GUYS I really need a little advice here!!


:confused: :eek: <img src="confused.gif" border="0">
 

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Size of the lines makes no difference whatsoever. I have no idea why they put different sized lines on cars, but for that matter, I can't for the life of me figure out why they don't standardize a lot of stuff like lug bolts, etc.

The only things that affect the pressure brake system is the ratio of the master cylinder diamater (small = high press , large = low press) to the wheel cylinder diameter (small = low press , large = high press) and the ratio of the distance from the foot pedal to the pedal pivot to the distance from the pivot to the master cylinder push rod (larger ratio = higher press for given foot pressure). All the lines do is transfer pressure. Could be 1" diameter lines or 1/16" diameter lines, makes no difference.

Adjustable proportioning valve usually goes to the rear whells, not the front. The valve reduces pressure to the rear wheels so they will not lock up before the front wheels lock up. Believe me, I know from experience that you don't want the rear wheels locked up and the front wheels rolling free!! When this happens, you get a very quick change in scenery - you get to see where you just came from.
 

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NYOFP4RJ3CHRIS
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Is that anything like pullin' the park brake lever while goin'round a corner? :D :D
 

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I would have to disagree about the size, with Hyro. the larger the surface area of the piston, the more force generated with the same input, so if you have larger wheel cyl, you will need more vloume of fluid to react on the piston, therefore, a smaller line, would require the pedal to be pushed further, to displace the same amount of fluid. Kinda like a 1 ton truck, the master Cyl looks the same as a half ton, but has larger pistons, to move more fluid for the larger wheel cyl, now if you had TINY lines, it would take alot of travel to move enough fluid, but a 1 inch line, would take Very little. So yes, in the big picture, I think that line size does matter, but in a normal aplication, prob not enough you would ever realize. ...if this sounds nuts, it's cause I just woke up.. lol, Yall have a Great day. :rolleyes:
 

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Wheelsup; That sounds intuitively correct but can't use intuition here. Think of the brake lines as a black box filled with a non-compressible fluid, which in fact it is. This box has rigid sides that can't flex which is the case with steel brake lines. Teh black box has two small openings which fluid can flow into and out of. By definition, we know nothing about the size of the box. Now inject 1 oz. of fluid into one of the holes in the box. Since the walls can't flex and the fluid is incompressible, that 1 oz. of fluid has no choice but to be ejected from the other hole in the black box. The size of the black box has no bearing on the fluid displacement, only the size of the injector (master cylinder piston in our case). The black box can be the size of the pacific ocean or a 1 oz. jar, the effect on the output from the input is the same. Obviously, no fluid is perfectly incompressible and no container is perfectly rigid but in a brake system the components are virtually that way. Thus, size of the brake lines has no bearing on the function of the brake system. The only conceivable effect would be on a huge truck where the pistons are so large that a lot of fluid is transferred. If the lines are too small that friction losses are high, the brakes will function sluggishly. Otherwise, don't be concerned about the size of the tubes, just get the ones that fit the fittings and they, big or little, will work fine.
 

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dude how did you get to over 1200 posts? anyway willys is absolutely correct, the only thing governing the pressure multiplication in a hydraulic system is the ratio between the areas of the pistons acting on the fluid

[ January 14, 2003: Message edited by: deuce_454 ]</p>
 

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Well, I will stand by my Theory that the size of the line, dictates the Amount of un compressable fluid that can travel through it, and in return, the amount of time / travel it takes for the fluid to react upon the pistons.

In your lil black box, what happens if you change the size of the output hole ? does it empty quicker ? does it allow the fluid to be put in faster ? A pin hole, would take a long time to flow enough fluid to act upon a brake system, as you are having to fill the Cyl. with fluid. now a 1 inch hole, would allow the Black box (we will say a 1 gallon capacity) to empty very quickly, and thus move a large volume of fluid through the system. So once again, I think that size matters.

Hmmm... I have a 1 ton Chevy with a twin Cyl dump bed, if I replaced the 1/2 inch id lines going to the rams with 3/16, would it have no effect on anything ? Hell yeahs it would, it would take All day to dump the bed, although it would dump eventually. This is not different than the Brakes.

Why is the single brake line going to the rear on a chevy twice the size as that for the front 2 ? would they not have just made it the same size to save money if the lil bitty line would work ? Hell yeah they would, but the line Won't work, cause size matters.

If you replaced the 3/16 line with 6 inch line, would it effect the brakeing ? Hell yeah it would, you would be lucky to stop unless you had a master cyl the size of a BBC motor, because the Velocity of the fluid would be Very slow.

These are Extream examples, but they show what I am saying, or at least I hope so, Maybe we can do a 3 way call with a few hydro companies and set it straight, as I feel I am right, and so do you. I hope there is no offense taken by this debate, it's all in good nature from this end, and I love to argue when I'm right....Hehehe Have a good one :rolleyes:
 

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NYOFP4RJ3CHRIS
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Originally posted by WheelsUpRacing:
[QB]If you replaced the 3/16 line with 6 inch line, would it effect the brakeing ? Hell yeah it would, you would be lucky to stop unless you had a master cyl the size of a BBC motor, because the Velocity of the fluid would be Very slow.

Velocity of the fluid would not be a problem if you went to a larger than needed line diameter. Therefore, if you went from 3/16 to 6 inch diameter line, it would have no effect whatsoever.

I see what youre saying about too small of a line diameter acting as an orifice, especially on something like a dump ram. But were not talking about such a drastic difference in diameter here, and not even close to the same volume of fluid. Generally rear drum brakes have 1/4" line to the rear axle, and then it splits to 3/16" diameter to each wheel cyl.


Pascals law states that pressure exerted on a confined liquid or fluid is transmitted undiminished and equally in all directions and acts with equal force in all directions. Therefore, Willy's is also correct in his statement.


You guys are talkin'bout two different things is all.


What I would do is, run 1/4" to the rear axle, and then 3/16" split off to each wheel cyl. I bet that's what the fittings will dictate as well.

:)
 

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Discussion Starter · #11 ·
DAMN THESE GUYS ARE GOOD! Oooops, that's golf isn't it? Anyhow, the reason I'm asking is that with the 1/4" lines split into to 3/16" lines to the back, my front discs did all the stopping. Yes I had all new drums, shoes, springs, etc. Adjustments were all kosher too. That's why I bought the adjustable proportioning valve, thinking I needed to restrict the flow to the front discs. The master cylinder is for a non-power brake cheby with disc/drum combo, so it should be right. Any thoughts? How about another good ARGUEMENT???? <img src="graemlins/boxing.gif" border="0" alt="[boxing]" />
 

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Bummer. I guess I need to go back to college and update my Bachelor of Science in Mechanical Engineering degree and un-design all the equipment I have designed over the past 30 years using my wrong ideas!

Seriously, the line sizes aren't pertinent. Sounds like you may have a master cylinder with problems. Front and rear lines are fed by separate rubber cups in the cylinder and the rear may be installed wrong or broken. Is the master cylinder new or used? How old is the kit in it? May just need a fresh kit in the master cylinder. Also, are the rear brakes not working at all or just not as much as the front? Front brakes do +60% of the stopping so shoes/pads should go out on them faster than the rear. You shouldn't need to put a pressure reducing valve in the front system under any curcumstance so I would spend some time investigating why the rears aren't up to snuff. May even need to (GAG!) take it to a shop! Can't believe I said that!!!!!!

[ January 14, 2003: Message edited by: [email protected] ]

[ January 14, 2003: Message edited by: [email protected] ]</p>
 

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funny you should say that willys, perhaps i could get my college tuition back too. i didnt know you were a fellow mechanical engineer. tell me then; Canadian engineers, is it true you all wear an iron pinkie finger ring made from steel from galloping betty?
 

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Discussion Starter · #17 ·
Everything new in the back drums, including drums. New master cylinder, okd calipers, new pads on new rotors in front. I adjusted the backs several times to make sure they were right, and bled the system three times. The master cylinder is above both the disc calipers, and the slave cylinders in the back. When I try to stop with any urgency at all, I can lock the fronts up, and get seemingly no action from the backs. When I power brake it, the fronts will lock up, and still spin the rear meats. I have pondered residual valves for front and rear, adjustable proportioning valve, which I have but haven't installed. Stumped, but haven't considered taking it to a [email protected]*P! I can't believe you said that either!! :eek:
 

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Discussion Starter · #18 ·
WHAT pressure valve needed in the rear brake lines???? :confused: :confused: <img src="confused.gif" border="0">
 

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Sure sounds like the master cylinder has a bad rear wheel plunger. I would tear it apart, make sure it doesn't have a badly pitted bore (common in used cycliders that have been idle for any time at all), then install a new kit to be sure that isn't the problem. If you have mixed and matched components (if it is a real hot rod, you better have or you loose your man card!), you may have mis-matched cylinder sizes. If the slave cylinders are too small in diameter compared to the master cylinder, you aren't generateing enough force in the rear brakes to establish the friction you need to stop the drums. If you have installed tires that are a lot taller than the brakes were designed for, that will make the brakes seem weak also.

Assuming the master cylinder is in good shape, and if you have a common rear end, go to your parts shop and see if they list larger diameter bore slave cylinders. They usually carry several diameters for common rear ends. Compare the diameters they list to the diameter of yours. If they carry sizes larger than yours, buy a couple of the largest and install them. This should get your rear brakes contributing. Then if they are locking up B4 the front, you can install your pressure regulating valve in the REAR system and all will be well in the universe!

Connman is probably talking about the resudual pressure check valve that should be build into the master cylinder if it was made for drum brakes. The valve maintains about 10psi to precharge the system and prevent air invasion which is required in drum brakes.
 

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go look at a lot of 70'-80' gm rear wheel drive cars a lot of them are 3/16 front 1/4 rear residual valve and proportioning valve [adjustible]should complete system make sure bleeders are below mc heigth to bleed
 
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