1958 GMC 1/2t truck Converting to disc up front. Bought a 6 lug disc brake kit Are the brake lines all the same size? 3/16? What are the residual valve sizes needed? red 10 and blue 2
3/16" tubing; 10lb red - Drum; and 2lb blue - Disc. Make sure that you get the correct master cylinder (Disc / drum) with adequate bore (1 1/8"). Personally I'd also use a booster due to pedal pressure.
I find 1/4" is better for disc, they say 3/16ths but having done a bunch I find the disc brake just work better whenever I ran the bigger line. The 'up' side is that you only need 1 size fittings, tubing etc to do the whole car.
Most residual valves are 1/8" NPT; so you have to use an adapter either way. Usually switch to 3/16" lines at the tee on the rear axle.
No. I work for an OEM, that is 100% disc, on every vehicle sold in the US. All lines are 3/16", in every single application. If 1/4" provided better braking, our legal department would require it. We have not used 1/4” since the 60's, before domestic discs were a common thing.
3/16 all around on every build for the last 40 years using cupronickel tube and double flare , simple as that . Just use reducing tube nuts when needed. 3/16 is nice and tidy easy to form .... 1/4” is for 18 wheelers .
Disagree Gimps, next time you do one then use 1/4" and then you'll know that it is better and here is why: contrary to popular belief disc brakes require MORE fluid to operate than drum. The surface area of a disc brake piston is much greater than the area of a wheel cylinder piston and therefore requires more fluid to operate - that is a physical fact, indisputable. It costs less to run 3/16ths and the car stops within requirements so there is no reason to run larger lines on professionally engineered systems. But I can tell you as a fact that every race car and street car I have plumbed with 1/4" lines the brakes have worked better, I have spent much less time sorting problems out, they assemble better, they bleed better, less troubleshooting and second guessing yourself.
The area of a disc brake piston is larger, but the travel to make the brakes apply is less. Both types of brakes use a similar volume of fluid. Somehow, all those modern cars built in the past 40-50 years with factory disc brakes seem to work fine with 3/16" tubing. Using 1/4" doesn't hurt anything, and was used on several drum brake applications in the old days.
Disagree, the disc uses greater fluid, why do you think the bowl area is larger for the disc than the drum? Because it takes more fluid to operate the system. Look at master cylinders, dual circuit MC's had the same size bowls for drum-drum, when the disc became standard on front the MC front bowl volume had to be increased, they just didn't feel like making it bigger and making new covers because they were bored. I realize I am saying some heretical shit here that goes against all popular wisdom but what I am saying comes from book learnin, the Harper-Collins classroom manuals for the certifications.
No. The reservoir of a disc system is larger for a very specific reason. The pistons in a drum system always return to the same place, but in a disc system, as the pads wear, the pistons return less. In normal use, fluid from the reservoir moves to the pistons, as the pads wear. If you use a small drum type reservoir with disc brakes, it would run out of fluid before the pads are worn out. I just have a mechanical engineering degree, my ASE certifications expired long ago...
With a larger tube you are moving more fluid, not creating more pressure. Disc brakes do not require more fluid to opperate, but they do require a larger reservoir to hold more fluid, because as the pads wear the caliper fills with fluid. Like Squirrel said if you used a small reservoir you would run out of fluid before your next brake job. Drum brake wheel cylinders always return to the same position. That’s why rum brakes require service and adjustment, and discs can be installed and forgotten about untill the next part replacement ( in an ideal situation) Not disputing a larger line makes bleeding a system easier as you are moving more fluid, but as a requirement I would disagree. Most new brake parts are set up for 3/16ths “ line and fittings It’s easier to bend and set up also. And as long as we are throwing credentials around 20 year red seal automotive mechanic had 6 of my ase certificates and 6 years in heavy equipment and material handling with a lot of time spent repairing and building hydraulic systems .
I think a test mule is in order? That said I won't dispute reservoir volume or caliber volume. But what about the restriction of fluid travel? This would only factor in depending at what rate a caliber can keep up with a wheel cly.?
The flow rate is really the only thing that will be affected by tube size. And 3/16" seems to work for antilock brakes, which work a lot faster than any human driver, eh?
So at each wheel where the flex line; the standard size is 3/16; same for the rear drums? so i will be good to buy all 3/16 parts.As i haven't removed the original brake lines.
The only thing I have to add is be careful which brand of residual valves you use, some have a tendency to leak. I've been using the SSBC ones with good luck (Wilwood not so much). Let us know how it goes, I'm thinking about going the manual disc route only if it improves braking. The manual disc that was on the '54 Ford panel when I bought it was OK, but the stock drums required less effort to stop.
The Jalopy Journal
