Brake line size, what is the best?

If the size of the line was really important, then the line on your rear axle (after the T) would be the same size as the line feeding the T. Most cars use 3/16" on the rear axle.
I use 3/16" all around, and I am also using a master designed for disc-disc on a disc drum setup. Just a prop valve to the rear to adjust the pressure.
The master will deliver the same amount of fluid to a drum or a disc. Difference is discs ride on the rotor, so less volume needed, and drums usually have more clearance, so the shoe travels a bit before contacting the drum.

 

There is a reason GM used both sizes. It involves flow rates, pressures and cylinder volumes. They are all related.

 

The 3/16" lines after the T carry only half the flow of the 1/4" line feeding the T, that's why they can be smaller diameter.

 

an engineering fact. If you push on the brake pedal with a certain pressure of your foot, the force on the brake pads will increase as you reduce the brake line size to the point where effective volume of fluid threshhold is reached..

 

Engineering fact. When you apply force to the brake pedal, the pressure generated in the master cylinder is the force applied to the master cylinder piston divided by the cross sectional area of the piston. ( lb/sq. in. or psi). This is the pressure of the brake fluid and the pressure in the calipers or wheel cylinders. The force generated by the caliper or wheel cylinder on the disc or shoes is this pressure multiplied by the area of the caliper or wheel cylinder piston.This does not matter what line size you have. The pressure will be the same no matter what line size you have.

Man I have to say, I've been a practicing mechanical engineer for over 30 years and if an engineer told you that it is an engineering fact that force on the brake pads INCREASES by simply DECREASING line size ,that engineer needs a refresher course in physics and fluid mechanics. The only thing smaller line size would do is to increase the pressure required in the master cylinder to move the fluid through the smaller line at the same rate.
If you want to increase the force on the pads you have to either increase the system pressure by pushing harder on the pedal , or if you can't do that then you have to increase the size ( area) of the caliper piston(s).
Goodbye!

 

wow,you guys will argue over everything,Hey Martha Stewarts making water from scratch!

 

Does that mean you could run 1/2" line and adapt it down to 1/8" line 2" ahead of the wheel cylinder to increase cylinder pressure? Kinda like power brakes without a booster? This could be an engineering brakethrough!

 

There is a difference between have a smaller bore master from a larger .I found useing a 7/8" bore my brakes feel like power when useing a 1" bore they reacted as a rock.I think it has to do with volume if you have a 1" hose and a given feed you get volume ,when useing same feed and 1/2" hose you get pressure.This is what I use and it works.I have drum rears big gm and 11" fullsize GM discs front Willwood 7/8" master.

 

I noticed we talked about sufficent volume so you dont have to pump or stroke your brakes to stop.I use a 2lb resiual valve for front and a 10lb for the rear thus holding brakes out to avoid haveing brake being pumped to stop car.I know it is a pratice when your master is under floor but it works for me.Joe

 

It is pressure, not volume, unless you go too small, in which case, volume becomes an issue.

Think of it this way. It is all about pounds-per-square inch. If you can manage to push with the same pedal force, a master cylinder with a smaller bore will produce a higher psi than one with a larger. Why? Because those pounds are expressed over fewer square inches.

Of course, in this case, you'd have to push the pedal just a little further to get full actuation, but that is not what makes the pedal feel, just the pedal travel.

 

The PO of my roadster (who should be banned from building anything) ran 1/4 in front and 3/16 in the rear to avoid using a proportioning valve. I do not recommend this as when I redo it all it will be 3/16 with both proportional and residual valves installed.

 

How about this: I have been building for about 30 years, and have used 3/16 (or the near equivalent metric size for Euro stuff) on all lines, on all vehicles where I replaced lines. Never had a problem yet, ever.

We can debate until the cows have been ground, grilled and eaten. Just use what is convenient for you, and you will be fine. Master cylinder size, residual valves, and proportioning valves are more critical than line size.

 

The reason the OEM's use larger lines to the rear is line loss - does not matter if disc or drum.

 

flip a coin, the coin knows all

 

While the 2 psi will keep the calipers from retracting any more than the run-out in the rotor, the 10 psi on the rear will not keep the wheel cylinders
extended against the force of the retract springs. The residual pressure does keep the wheel cylinder cups expanded. Proper adjustment either automatic
or manual is the only thing that maintains pedal level with drum brakes.

 

Yeah Iam good at over thinking this stuff I have a bad habit for trying to re ivent the wheel so to speak...LOL, You have a good point about the line size and pressure.

 

I like hearing different views on line size,I don't believe its any different useing 1/4 to rear,I also use a adjustable propotioning valve,I feel its to better dial in the brake bias when useing different size wheels.A stock proportion valve is just what it said Stock a car with the wheels that came with the car which was engineered by the factory.I don,t and never had any thing stock bike or car.