I think the only time you should change line size is when the fluid path splits, for example, 1/4 from the mc to the T on the rearend, then 3/16 to the wheel cylinders. That makes logical sence to me to equalize volume, pressure. I am interested to hear any thought on this too, I am going to be doing this soon.That should be true Jim, unless for some reason, the rear brake line was run down the right hand frame rail. I have seen stranger things in some builds. Oh, btw. what are thoughts on mixing 3/16" and 1/4" brake lines? In my mind this would cause a problem volume wise.
Lee
I know from basic plumbing that reducing line size increases pressure, but I don't think that would necessarily hold true in a braking system because volume is required to move the caliper, cylinder, etc. perhaps to regulate more volume to one and less volume to the other, ie disc/drum it may be a consideration. It seems that a smaller line would inhibit if the master was large enough to push adequate fluid volume. I plan on using a adjustable pro portioning valve and not worrying about line size differential. I " think" that running a larger line from the mc to the splitter, or "t", would enhance response. I've noticed that's how it's done on heavier vehicles that I have worked on. I am not sure if it would be noticeable in a light car like a T bucket. Time for me to do some internet research on hydraulics and fluid dynamics maybe? Lol.That sounds reasonable but I have never used 1/4" line on anything.
Ok, after a quick study session, it seems the big factor is mc bore vs wheel cylinder bore, not so much line size. Rear drums require more fluid than do disc, so larger lines are typically ran for them, the down side is it takes more fluid, ie more pedal travel to do the work, also they say larger line size can allow softer pedal due to fluid and line expansion. Wilwood recommends 3/16" hard line all the way the the flex lines, the flex lines should be as short as possible to minimize expansion, spongy pedal.I know from basic plumbing that reducing line size increases pressure, but I don't think that would necessarily hold true in a braking system because volume is required to move the caliper, cylinder, etc. perhaps to regulate more volume to one and less volume to the other, ie disc/drum it may be a consideration. It seems that a smaller line would inhibit if the master was large enough to push adequate fluid volume. I plan on using a adjustable pro portioning valve and not worrying about line size differential. I " think" that running a larger line from the mc to the splitter, or "t", would enhance response. I've noticed that's how it's done on heavier vehicles that I have worked on. I am not sure if it would be noticeable in a light car like a T bucket. Time for me to do some internet research on hydraulics and fluid dynamics maybe? Lol.
From what I read, the larger line requires more volume, and thus requires more pedal travel. There is a formula, I dont remember it right off. I think it would depend on more than just the line size," think", because there are variables, such as the master bore, wether it's disc or drum, pro portioning valve bias, etc. from what wilwoods site said, they recommend 3/16" and stated it provided firmer pedal feel and less pedal travel. Another site I looked at explained that cars with rear drum brakes used 1/4" line from the master to the " t", then 3/16" to the wheel cylinders. They explained that the drums required more fluid to move them than disc, so the larger line provides the volume, again that is from the master back to the split. Wilwood stated that larger line size allowed softer pedal due to fluid expansion and line expansion, I wouldn't have thought that would be an issue, they also instruct to use the shortest possible soft or braided line for the same reason. Also larger line requires more pedal travel because you are moving more fluid. I believe Wilwood was speaking more to disc than drum brakes. So the example of your side car proves the theory, sort of, if you think about changing the line to a larger size after the split, you in effect created a larger area, which required more volume, so the brake reacted less aggressively. That was a smart fix! I purchased a master designed for disc/ drums, so I plan on plumbing my brakes as the manufacture did with larger line to the rear drum split, then smaller to the wheel cylinders and small line to the front calipers. I hope the adjustable pro portioning valve and the residual valves are enough to make it workout..... That brings another thought to mind, could the excessive pedal travel be caused by not having residual pressure valves installed and allowing to much fluid travel?So, the line change to larger, wouldn't make any difference? How about the extra volume needed to fill the 1/4" versus the 3/16" when the 1/4" is the jump line between calipers? No argument, just wanting to understand. Once when working on a motorcycle with a side car with disc brake on all 3 wheels, the side car wheel would lockup. I changed the 3/16" line to the side car to 1/4", and it solved the lockup problem. Front wheel and side car wheels were fed off front brake on bike.