What Are the Criteria for Sizing Hydraulic Brake Lines?

[jack action]

On older GM cars, the brake lines are 3/16" from the front port of the master cylinder, splitting into two 3/16" for each front wheel, and 1/4" from the rear port of the master cylinder, splitting into two 3/16" for each rear wheel. This is the case for drum/drum or disc/drum configurations.

I'm guessing the larger diameter of the rear line has something to do with the longer length of the line, but I can't find any reliable info on that. What is the advantage of 1/4" over 3/16"? I know some people replace everything with 3/16" all around and don't seem to have any problems. What would happen if one went down to, say, 1/8"?

On a more general note, I wonder what are the criteria for hydraulic line sizing with a short-stroke piston arrangement like with brakes or clutches actuators? Is it still flow velocity like with a pump?

 

[jim hardy]

Maybe it was so an inattentive technician wouldn't get them crossed ? Old GM's had a proportioning valve. I'm under the impression that valve delays rear wheel fluid pressure to avoid rear wheel lockup.

 

[Ranger Mike]

The introduction of disc brakes was a big deal in the 1960s. Back then your passenger engine could weigh in at 650 to 750 pounds. So the front engine car was enjoying 75% front end weight under braking. On disc brakes you had a vacuum booster to up the pedal pressure so we had 100 pound foot pedal force multiplied to 1300 pound force from the master cylinder. You needed this to squeeze the single piston located in the front brake caliber to stop rotation of the disc.

Since the rears were drum brakes and the wheel cylinder had a longer stroke vs. front caliber pistons, this meant a larger line to get the desired stopping effect. The manufacturers added an in line pressure reducer ( residual pressure valve and an in line pressure proportioning valve) to the larger rear brake line. Ifin you had 1300 psi on the rear wheel cylinder it would blow, big time. We run an adjustable proportioning valve on the race car to fine tune the brakes so as not to lock up the rears before the fronts.

 

[jack action]

Maybe it was so an inattentive technician wouldn't get them crossed ?

That could explain the line from the master to the distribution block, but not the one from the distribution block to the rear wheels (which is impossible to mix up with another line). Actually, come to think of it, both lines have fittings of different size at the master cylinder just to solve that problem, so a different line size wouldn't had anything.

Since the rears were drum brakes and the wheel cylinder had a longer stroke vs. front caliber pistons, this meant a larger line to get the desired stopping effect.

What is your scientific basis to state «this meant a larger line»? That is what I'm looking for. And remember, car with drums all around still have the larger line to the rear, even though the front wheel cylinders are larger than the rear ones.

 

[Ranger Mike]

Jack, once again you make me think...and i have to fine tune my reply..thank you...

The governing equations for this type of system are based on Pascal's Principle, which states: "Pressure is transmitted undiminished in an enclosed static fluid." And, although brake fluid does move somewhat, it's basically in a static state. It moves a bit, but very slowly. The relevant equations are:

Fw = (Aw x Fm) / Am, where:

Fm = force applied by the master cylinder piston (lbs, not psi)
Am = area of the master cylinder bire
Fw = force applied by the wheel cylinder piston (psi)
Aw = area of the wheel cylinder bore

and,

Xw = (Xm x Am) / Aw, where:

Xw = distance traveled by the wheel cylinder piston
Xm = distance traveled by the master cylinder piston

Using these two equations, if the wheel cylinder bore is 4 times the area of the master cylinder bore it will apply 4 times the force, but the master cylinder piston must move 4 times as far.

Note that the volume of fluid in the system has nothing to do with either calculation. Assuming that the system is full, the master cylinder piston is going to move the same distance with the same pedal stroke, and it's going to exert the same pressure on the wheel cylinder piston, regardless of line size.
The real reason the Big 3 ran ¼” rear brake line.

Years ago I was talking to a tool and die maker for Bundy Tubing in Detroit. He built the tooling that mass produced the bends in the tubing for the big 3 back before they used computer controls (he made different profiles for the cams and switches used in the machines). Cadillac and Chrysler because they used 1/4" line because that didn't have to have as many clamps on the lines because the larger lines didn't vibrate as much as smaller brake lines. The bends weren't as critical to get the line in place and were a lot more forgiving due to the beefier construction.

The smaller 3/16" lines that Chevy and Ford used, although they saved money on the actual lines, had to use more clamps and had more critical bends because they could vibrate big time over long distances. Vibration is a killer on race cars and even street cars over time.

 

[jack action]

Regarding the area ratio, that is where I'm baffled: In theory, the line size between the cylinders is irrelevant, it could be as small as one desired. In practice, we know there should be some minimum value. Do we look at fluid velocity (even though there is very little motion)? Are we looking at laminar and turbulent flow? Is it a tube strength issue? Is it a heat sink effect to cool the fluid?

Cadillac and Chrysler because they used 1/4" line because that didn't have to have as many clamps on the lines because the larger lines didn't vibrate as much as smaller brake lines. The bends weren't as critical to get the line in place and were a lot more forgiving due to the beefier construction.

The smaller 3/16" lines that Chevy and Ford used, although they saved money on the actual lines, had to use more clamps and had more critical bends because they could vibrate big time over long distances.

That is interesting. But there must be something else as both lines going from the master cylinder to the distribution block are of different sizes even though they basically follow the same path. If saving money with a smaller line was so important, they surely wouldn't be making one larger than the other just for the fun of it.

 

[Ranger Mike]

the 1/4 inch line costs more per foot but takes less labor time on assembly line. It has a longer life due to less vibration compared to smaller line. The smaller line requires many more clamps and time to attach the clamps. You do have different pressure coming from the master cylinder ,A disc/drum master cylinder has 1 large and 1 small reservoir and built in residual valve on the drum side. Disc/disc has the same size reservoir. Pretty sure the pressure is same in both but …rear disc normally has smaller pistons which require less pressure. I do know the disc/drum master cylinder has larger piston for the front and smaller piston for rear drum which would change the rear pressure. See attached pic.

brake fluid dot 3 will usually prevent brake fade ( fluid boils and spongy pedal due to moisture present in the fluid) on passenger cars. Heat sink thing happens on race cars and that is another issue.

 

[jim hardy]

I had a '76 Maverick front disc/rear drum with no power assist. Always stopped okay though i wondered how.

Different bores in the master cylinder , perhaps ?