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Discussion Starter #1
So what is the size of the tubing and the fittings that run from the master cyl to the distribution block on a 1971 with manual brakes?
 

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Paul
Years ago when I was making up my master cylinder bleeder kit, I had the same question. I went to NAPA and bought the metric conversion brake line because that is what fit my M/C. I don't recall the size but they weren't std on my 72.
 

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Master
Front brakes 1/2-20 3/16" line
Rear brakes 9/16-18 1/4" line

Dist Block
Front brakes are 1/2-20 flare nut for 3/16" line
Rear brakes are 7/16-24 flare nut for 1/4" line
 

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Discussion Starter #4
So did GM use different tubing sizes and fitting sizes just so people would not mix up the front and rear brakes or is there another reason?
 

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So did GM use different tubing sizes and fitting sizes just so people would not mix up the front and rear brakes or is there another reason?

The reason is bias. The braking system is not split symmetrically. The front brakes need more than the rear brakes.

Since there is only one master with a specific bore size they had to control the front to rear bias another way.

With different tubing sizes you control the braking pressure applied when you step on the pedal.
 

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The reason is bias. The braking system is not split symmetrically. The front brakes need more than the rear brakes.
And that's why the fronts use the smaller lines, because they need more??? :smack

Since there is only one master with a specific bore size they had to control the front to rear bias another way.

With different tubing sizes you control the braking pressure applied when you step on the pedal.


That's 100% bullshit. :crazy:
In hydraulics, if you apply 100psi at one cylinder (master), the fluid pressure at the other end cylinder (caliper) will also be 100psi.
Similarly, if you move 1 oz of fluid at one end, you will move 1 oz of fluid at the other end.
Doesn't matter if you have a 1/8"dia tube or and 8"dia tube.
In C3's that use a distribution block, the braking bias is controlled by by different sized pistons in front vs rear calipers, relating to different clamping forces. Nothing to do with tubing.
I'ts Basic hydraulics.
There are different control valves available to change flow and pressure.

The OP's guess what correct. :thumbsup:
 

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That's 100% bullshit. :crazy:
In hydraulics, if you apply 100psi at one cylinder (master), the fluid pressure at the other end cylinder (caliper) will also be 100psi.
Similarly, if you move 1 oz of fluid at one end, you will move 1 oz of fluid at the other end.
Doesn't matter if you have a 1/8"dia tube or and 8"dia tube.
In C3's that use a distribution block, the braking bias is controlled by by different sized pistons in front vs rear calipers, relating to different clamping forces. Nothing to do with tubing.
I'ts Basic hydraulics.
There are different control valves available to change flow and pressure.

The OP's guess what correct. :thumbsup:

Terribly sorry I offended you. I asked the same question a few times and that's the answer I was given. The tubing size contributed to the control of the bias front to rear.

I'm not sure about the piston size change though, since the front calipers are the same on cars that have dist. blocks and cars that don't. If the dist. block is the method of control, and some don't have one, it would fall completely on the pistons. But if all front calipers are the same???

Also, why would GM go to the trouble of two different tubing diameters if it didn't make any difference?

Tubing size does matter in hydraulics, just not in regards to pressure. Example:

Your master cylinder has 100 ml of fluid. (Example, not exact.)
One set of lines are 3/4". The other lines are 1/4". You evacuate your master completely when you press the pedal.

The fluid split won't be 50 / 50. In fact it will be 75 / 25 in favor of the larger diameter lines.

You're telling me that makes no difference?
 

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Terribly sorry I offended you. I asked the same question a few times and that's the answer I was given. The tubing size contributed to the control of the bias front to rear.
No need to try to bring it to a personal level. Just correcting your response. ;)

I'm not sure about the piston size change though, since the front calipers are the same on cars that have dist. blocks and cars that don't. If the dist. block is the method of control, and some don't have one, it would fall completely on the pistons. But if all front calipers are the same???
The dist block (actually a pressure differential valve/with switch) has no bias control whatsoever. It simply keeps the front and rear hydraulic systems separate in all positions and the switch lites a warning bulb.

Also, why would GM go to the trouble of two different tubing diameters if it didn't make any difference?
Because they didn't want people that don't understand the system to screw it up. For the most part, other than a very few fittings, it also allowed off the shelf standard SAE parts to be used.


Tubing size does matter in hydraulics, just not in regards to pressure. Example:

Your master cylinder has 100 ml of fluid. (Example, not exact.)
One set of lines are 3/4". The other lines are 1/4". You evacuate your master completely when you press the pedal.

The fluid split won't be 50 / 50. In fact it will be 75 / 25 in favor of the larger diameter lines.

You're telling me that makes no difference?
100% wrong again. It's a closed system.
Reread my original post. You must understand basic hydraulic theory (pressure, flow and volume) or you will remain confused with a hydraulic brake system
There are plenty of simple primers for hydraulics to be found with google.
 

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No no, it all makes sense but contradicts what someone else told me...kind of in the same way, when I originally asked many years ago.

He worded it just like you are except that he made it clear that tube diameter from the master had a specific purpose.

Made sense then and I guess I never questioned the tubing size aspect of the system.
 

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Similarly, if you move 1 oz of fluid at one end, you will move 1 oz of fluid at the other end.
Doesn't matter if you have a 1/8"dia tube or and 8"dia tube.
one thing to note... that 1 oz of fluid moving through a 3/16" line will move piston at the other end FARTHER than the same 1 oz of fluid moving through a 1/4" line. So we can assume the front pads will contact the rotor sooner than the rear pads. And because there is very little pressure that builds in that line until the piston stops moving, the pressure will more than likely be greater at the front than the rear, correct?

If that's the case, the line diameter would have an affect on brake bias.

It's also important to note that the M/C on these cars is a dual reservoir system, so that hypothetical 100 mL of fluid is not shared by the front and rear brakes. The front brakes have their dedicated 50 mL and the rears have their dedicated 50 mL.
 

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one thing to note... that 1 oz of fluid moving through a 3/16" line will move piston at the other end FARTHER than the same 1 oz of fluid moving through a 1/4" line. So we can assume the front pads will contact the rotor sooner than the rear pads. And because there is very little pressure that builds in that line until the piston stops moving, the pressure will more than likely be greater at the front than the rear, correct?

That's an incorrect premise.
Given: All pistons are the same size.
1oz at one end still ends up being 1 oz at the other end so both pistons move the same amount. Simple displacement.
Only the velocity of the fluid will increase travelling thru the smaller line. GPM or flow will remain the same from both sections of a tandem master cylinder.
To move the piston at the other end it takes fluid volume only.

If that's the case, the line diameter would have an affect on brake bias.

It's also important to note that the M/C on these cars is a dual reservoir system, so that hypothetical 100 mL of fluid is not shared by the front and rear brakes. The front brakes have their dedicated 50 mL and the rears have their dedicated 50 mL.
Hope that helps.
 

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That's an incorrect premise.
touche. I wasn't considering the entire system. I was only envisioning the brake lines where an equal volume would be two different lengths in two lines with different cross sections/diameters. But as you point out, the only diameter that affects piston travel is the piston diameter. If the pistons were the same size in front and rear then they would both move equal distances. Makes sense now.

It also makes sense that they put larger pistons in the front calipers than in the rear. I believe they were 1-7/8" in front and 1-3/8" rear? Did only the heavier cars of the era get the 2-1/16" pistons?
 

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I always thought that the smaller lines on the front were that way to allow more fluid(volume) to the larger Pistons of the front calipers since the smaller line holds less(volume)?

If the front and rear lines where the same size, the MC spool displaced the same amount(volume) to the front and rear, and the front pistons are a bigger dia wouldn't they have less travel?

Wouldn't the larger line of the rear brakes compensate for the travel of the smaller pistons in the rear the same way?
 

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Easy to picture

Picture this - master cylinder hooked to a half inch line on one side, and a two inch line on the other.... both lines are facing straight up and are FULL, as is the master cylinder.... on top of each line is a fitting attached to a glass tube.... when fluid is fed equally into each line, the glass tubes will also fill equally - basically a full brake line + 1 ounce is still a full brake line + 1 ounce no matter what the diameter of the line
 

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Whheeeewww! There has been alot of figuring going on in here.

Build the new lines like the old lines, even the loops if they have them, or you will have a difference in braking.

The line size and the routing along with any loops (loops in places for no reason) all work in conjunction with proportioning and plunger size to give the car the right braking.

Don't change anything, just build the line exactly like th egg heads at GM did. They already did all the figuring for you.

The only reason to change the system is if you are building it for track use and want to customize the braking for your set up or driving style.

:partyon:
 

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The loops in a hydraulic line are to prevent cracking. Any line, steel, rubber, unobtainium or whatever, when you apply pressure to it will slightly stretch or expand. With a direct run from A to B, there's no place for that. With a loop, there's always a little bit of flex.
 
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The loops in a hydraulic line are to prevent cracking. Any line, steel, rubber, unobtainium or whatever, when you apply pressure to it will slightly stretch or expand. With a direct run from A to B, there's no place for that. With a loop, there's always a little bit of flex.
You egg heads are too smart! See, I knew there was a good reason! :laughing:
 

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I'm not that smart. Just 30+ years of big airplanes. If I was smart, I'd be retired or making $150k a year and working when I felt like it.

:smack
 

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I'm not that smart. Just 30+ years of big airplanes. If I was smart, I'd be retired or making $150k a year and working when I felt like it.

:smack
:laughing: Well here's a paddle since we are in the same boat together. Might as well row together too.
:cheers:
 
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