What size brake lines?

[ltdmstr]

So I'm doing a manual to power brake conversion with hydroboost, as well as front disc conversion, on a '68 C10. I'm also replacing all the brake lines, front and rear. I did a search on here to see what size brake lines to use, and the results are just about every conceivable combination of 3/16 and 1/4 inch. Is there any consensus as to what size lines to use from the mc to the junction, from the junction to the front discs, along the frame rail to the rear flex line, and from the rear flex line to the rear drums? Thanks.

[Rickysnickers]

My truck is four wheel disc and all the lines are 3/16". I don't know if that helps you or not.

[ltdmstr]

Leaning toward 3/16 at the ends, front and rear, with everything else 1/4. Not sure if it makes a difference, really.

[jgr]

It depends on the bore on the master cylinder, You need to start first with knowing that then go from there. I would call tech line at someplace like Wilwood. Most lines are 3/16 BUT

[ltdmstr]

Using a Corvette-style master cylinder with 1-1/8" bore. Delco part number 18030333.

[jocko]

No need to look any further than how GM did it. Stock GM disk/drum setup is 3/16 for disk circuit, 1/4 for drum circuit.
Same logic holds true for 4-wheel disk (all 3/16 lines) or all drum (all 1/4 lines).

[ltdmstr]

Originally Posted by jocko

No need to look any further than how GM did it. Stock GM disk/drum setup is 3/16 for disk circuit, 1/4 for drum circuit.
Same logic holds true for 4-wheel disk (all 3/16 lines) or all drum (all 1/4 lines).

Seems reasonable to me.

[ltdmstr]

Ok, so went to install some 1/4" brake line on the rear, but the fittings are 7/16" and the wheel cylinder threads are 3/8"! Not sure how I'm supposed to make that work. So unless i'm missing something, it looks like it's going to have to be 3/16" line (with 3/8" fittings) all around.

[jocko]

Did you replace your 68's master cylinder with a disk/drum mc (either stock 71 GM truck or newer or an aftermarket one)? You do sometimes need to get line adapters at the mc (or at least I've had to in the past when buying a new mc)

[ltdmstr]

Originally Posted by jocko

Did you replace your 68's master cylinder with a disk/drum mc (either stock 71 GM truck or newer or an aftermarket one)?

I'm installing front discs, plus adding a hydro-boost (stock brakes were manual drums all around) with a Corvette-style master cylinder that has a 1-1/8" bore. Mc is Delco part number 18030333.

[jocko]

Not sure what vette style mc means - but most vettes were 4-wheel disk. You need a disk/drum mc if you are still running rear drums. That delco mc may work, but I don't know if it's a disk/disk or disk/drum mc.

[HO455]

Originally Posted by ltdmstr

Ok, so went to install some 1/4" brake line on the rear, but the fittings are 7/16" and the wheel cylinder threads are 3/8"! Not sure how I'm supposed to make that work. So unless i'm missing something, it looks like it's going to have to be 3/16" line (with 3/8" fittings) all around.

GM made the lines idiot proof so they could only be installed one way. The tubing from the hose to the wheel end is 3/16. The wheel end uses a regular 3/16 tube fitting (3/8 -24).
The end at the tee uses a fitting that is larger on the od (7/16-24) and the center ID is drilled for 3/16 tube.

https://www.jegs.com/i/RPC/707/R3590/10002/-1

Here is a link to my repair thread scroll to post starting at 194 for my brake line replacement. You may find some information to help you.

https://67-72chevytrucks.com/vboard/...=698377&page=8

If you are using the common replacement Corvette master cylinder you will need to obtain several other GM only sized tube fittings to hook the lines up. If you post a photo of the master cylinder you are using we should be able to help you get the lines there sorted out.

[ltdmstr]

Thanks for the info. Very helpfull. The t-fitting I have is also 3/8-24 thread on two sides, so it fits the 3/16 lines with the standard ends. Came from LMC. Here's a photo of the mc.

[HO455]

This thread has several ways shown of how to hook up the master cylinder to the junction block. I have installed these several ways (1st and 2nd photos for examples) depending on how the different components lined up and or needed to mounted. So there is no one correct way to do it. Just commonly used setups that all achieve the same result.

https://67-72chevytrucks.com/vboard/...d.php?t=711288

Hard to be sure from the photo are the two ports the same size or is the back one larger?

[ltdmstr]

Not sure if they're the same size or not. I'll check.

[ltdmstr]

Checked the mc and the front reservoir is significantly larger capacity than the rear. So I guess that means it's disc/drum.

[siggyfreud]

Since you're doing new lines anyways you might consider plumbing in an adjustable proportioning valve below the m/c.

[ltdmstr]

Originally Posted by siggyfreud

Since you're doing new lines anyways you might consider plumbing in an adjustable proportioning valve below the m/c.

I'm using this one from Wilwood.

[siggyfreud]

Perfect!