Hello, I have a previously installed power front disc conversion done on my 67 GS. The new MC and combo valve was hooked up with unions. I don't like that, and I want to run new lines. I need to reroute anyway due to interference from my newly installed headers. Is 10 feet of 1/4" line enough to run out of the combination valve down to the block out to the back hose? Alternatively, I already bought 25 feet of 3/16" line. Could I just plumb the entire system with that? If it would work safely, that would be my preferred option. I would rather avoid buying more tubing and importing it if I don't need to. I understand that the factory used 1/4" for the drums. Thanks for your input.
I just checked a repo 65-66 GS line I have, it's about 10' 5". 64-6 used 3/16" line, 67 went to a 1/4" line for the front-back line. I never heard an explanation of the engineering behind the larger ID, nor do I have a valid explanation. 3/16" will likely work fine in your 67 but you may need adapters at the combo valve and rear brake hose junction. If you can get 11' of 1/4" line, that would be preferred. On my blue 66, I added front disks and the combo valve was mounted several inches rearward of the original junction block. 10' of line would likely be enough. 66 had the junction block mounted on the frame just behind the upper A-arm. If you have the combo valve by the master cylinder, you'll need closer to 12' of line. I don't see a problem with using unions if the lines are correctly double-flared.
In an automotive hydraulic brake system, the line size (diameter and length) concern is that enough volume in the reservoir and the master cylinder's ability to fully and adequately pressurize and supply all the caliper piston's needs. As long as the total volume is considered, the line(s) diameter is not a significant issue.
The only advantages I can think of to the larger 1/4" line are: - a dent, kink, or rust in the line is less likely to restrict flow. - Since there is a larger volume of fluid, it can tolerate a little more water absorption. GM must have had a reason for the increase in size, wish I knew what it was....
