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View Full Version : Hard brake line size difference front/rear


turtlevette
09-21-2011, 12:17 AM
I've been working on the brakes for my old Surburban and it got me wondering about why the rear brake lines are larger diameter than the front. For instance on the burb, it has 3/16 front and 1/4 to the back Why not run 1/4 inch everywhere? I'm thinking its because 1/4 expands a fraction under heavy pressure causing spongy pedal.

The reason for 1/4? Because of the longer distance? OR Because you need 1/4 to feed 2, 3/16 pipes at the rear? With the kind of pressure generated you'd think intuitively it wouldn't really make that much difference.

mrvette
09-21-2011, 12:26 AM
I been scratching my ass over that one for decades.....and on one car I actually changed the line diameter, not that I can remember WHY.....

and it worked the same.....

so hoo, hoo, whooooo, nose.....


:beer:

denpo
09-21-2011, 04:32 AM
I'm no way expert on the subject, but I read somewhere in the internet it has to do with the pressure the line can deliver to the caliper.
The smaller diameter the bigger pressure.
Since most of the braking power has to go to the front, the line is smaller.

turtlevette
09-21-2011, 04:46 AM
I'm no way expert on the subject, but I read somewhere in the internet it has to do with the pressure the line can deliver to the caliper.
The smaller diameter the bigger pressure.
Since most of the braking power has to go to the front, the line is smaller.

I really appreciate your answer, but i'm going to shoot it down by saying the pressure is about the same with larger front piston area alone creating more clamping force.

It's OK we're brainstorming.

denpo
09-21-2011, 05:22 AM
I'm no way expert on the subject, but I read somewhere in the internet it has to do with the pressure the line can deliver to the caliper.
The smaller diameter the bigger pressure.
Since most of the braking power has to go to the front, the line is smaller.

I really appreciate your answer, but i'm going to shoot it down by saying the pressure is about the same with larger front piston area alone creating more clamping force.

It's OK we're brainstorming.

Thinking twice about what I said and realize you're right, brake line diameter does not change anything about pressure. Anyway I couldn't find the reference about what I previously said.
Here's another take. Looks like there was some kind of convention on using 1/4" lines for drum brakes, and 3/16" for disk brake. Would the line difference comes from the time corvette had front disk and read drum?
Maybe when they switched to 4 discs they just wanted to keep using the same MC.

mrvette
09-21-2011, 08:39 AM
I tell what drive me nutz too is GM's propensity to use different size line nutz for every damn one of the lines, sometimes even on different ends of the same line....think the thing was a diode or something only conducted one way or something...WTF?? so I have had a double flaring kit for decades, cut the damn nutz off the new line, reuse the damn GM nutz and have to reflare one end of the line....PIA for no GOOD reason that I can tell.....

:search::beer:

Twin_Turbo
09-21-2011, 11:22 AM
It has to do with pedal feel and pressure loss due to friction in the lines, the larger line is there because it has to feed 2 calipers in the rear over a good distance, instead of the fronts that have a dedicated line. The rear brakes should grab quickly for stability, not so much for braking power and as such a larger line can more quickly move the required volume with less friction. A 3/16 line would work but the pedal feel would probably be different. Otherwise in a closed hydraulic system the pressure is only dependant on the master cyl to brake piston ratio and the MC input pressure (foot pressure over pedal ratio)

clutchdust
09-23-2011, 06:54 AM
What TT said. There should be a single common line that runs to the back that is larger until it gets to the block that splits it off to the left and right calipers or wheel cylinders. At that point, the lines should be about the same size as the front lines.
The front lines should be individual coming out of your proportioning block to each side and will typically be one or two sizes smaller than the trunk line to the rear.
Another factor you have to consider is fluid volume. Now here's where I go out and make a big ASSUMEption on my part, but I'm guessing your Suburban has drums in the rear. If that's the case, you have to factor in that in a conventional set up, you are effectively operating four hydraulic cylinders in the rear to the normal two in the front because the wheel cylinders are basically just two opposing pistons on each side. I don't know the volumes in question but there is likely an increased volume requirement in the rear to effect the desired amount of shoe movement compared with the pad movement in the front. Don't forget the front pads simply glide on the disc surface and hence the pads never retract off disc save for a very small air gap. As opposed to the rear shoes which return to a mechanical stop (adjustment position) by spring pressure. That's why to get proper brake bias in a disc/drum configuration you need to either manually adjust the shoes regularly or make sure the auto adjusters work correctly.

turtlevette
09-24-2011, 03:23 AM
What TT said. There should be a single common line that runs to the back that is larger until it gets to the block that splits it off to the left and right calipers or wheel cylinders. At that point, the lines should be about the same size as the front lines.
The front lines should be individual coming out of your proportioning block to each side and will typically be one or two sizes smaller than the trunk line to the rear.
Another factor you have to consider is fluid volume. Now here's where I go out and make a big ASSUMEption on my part, but I'm guessing your Suburban has drums in the rear. If that's the case, you have to factor in that in a conventional set up, you are effectively operating four hydraulic cylinders in the rear to the normal two in the front because the wheel cylinders are basically just two opposing pistons on each side. I don't know the volumes in question but there is likely an increased volume requirement in the rear to effect the desired amount of shoe movement compared with the pad movement in the front. Don't forget the front pads simply glide on the disc surface and hence the pads never retract off disc save for a very small air gap. As opposed to the rear shoes which return to a mechanical stop (adjustment position) by spring pressure. That's why to get proper brake bias in a disc/drum configuration you need to either manually adjust the shoes regularly or make sure the auto adjusters work correctly.

Well....

I'm not sure any of us are right. The fronts are usually bigger pistons and require more volume and have to move it all through one 3/16 tube. The length of the tube is not that big of a deal when you're talking 1000psi. I would have to find a fluids book to do the calcs. The rear calipers or wheel cylinders normally are much smaller which kinda blows some of this theory out of the water.

You are wrong about the rear drums. There is a residual pressure valve built into the master cylinder that holds pressure on the wheel cylinder which keeps the drum shoes just barely touching the drum. That's one thing that i think rebuilders miss in the rear circuit. I think my rebuilt m/c has no or a defective residual valve which is giving me a long pedal. I'm going to install the red Wilwood valve just to be sure.

The adjusters make sure the shoes wear evenly.

I have the Wilwood blue (2 lb) in the front circuit on the vette to prevent piston knockback. What the springs were supposed to do with the stock calipers. The residual pressure valves make sure you get the quick grab TT is talking about and overall reduces the amount (distance) you need to press the pedal to get braking.