I apologize in advance for the size of this post. I’m throwing a lot out there for discussion. The reason is because I did quite a bit of reading on this subject before posting this, and I found a lot of conflicting information on both forums and in tech articles alike. I am planning to replace all my brake lines.

So, the first question is, do I use 3/16” line, ¼”, or a combination? (read on before answering!)

I am running discs on the front with  GM D52 calipers. (single piston) The rear will be a 2012 Colorado drum rear. I have a 1” Corvette style 4 port “universal” master cylinder, a proportioning valve in rear line only, and 2lb and 10lb inline residual valves. I’m running the MC under the floor in stock location, manual brakes.

It seems to me my options are: 3/16” line for everything, 3/16 to front with ¼ to a T at the rear, then 3/16” to the drums, or ¼” everywhere except after the T to the drums? (I think I have to run 3/16 directly to the drums due to metric wheel cylinder fittings. I haven’t found those fittings for ¼” line but I could be wrong)

The next question is, with all other things being equal, would 3/16” line result in less pedal travel than ¼” line? I’m not a fluid dynamics expert, but it seems to me it would, as any given volume of fluid would travel a further distance in a smaller tube than it would in a larger one. (I’m talking DISTANCE traveled, not pressure) I’ll get to the reason I’m asking below…

Another question is, should I use a 1 1/8” M.C.? The reason I’m asking this is also below.

Here's why I'm asking all of these things...

When I recently installed the disc conversion on the front, I was hoping to be able to shorten the master cylinder pushrod to lower the brake pedal a bit so I didn’t have to lift my heel off the floor to use the brakes. With the stock brakes, the pedal didn’t come anywhere close to the floor, so I though maybe the pedal didn’t need to be so high above the floor.

What I got was exactly the opposite. I was running the discs on front, with the original Huck drums on the rear. The pedal traveled virtually all the way to the floor, and was still somewhat soft. I could get the rear drums to lock up, but I could not get the fronts to lock no matter what I did. It was safe enough to drive short runs into town on our rural roads at or below 40mph, but I don’t think I’d want to be at higher speeds or on busier roads with it.

I know what you’re thinking...air in the system. I bled all four corners multiple times. Then I bled them some more. I kept bleeding LONG after I saw the last bubble come out each line. I went through 3 large bottles of brake fluid. (I didn’t reuse any fluid that was run through, as I was also flushing out the old fluid and lines) If there was any air left, I sure don’t know where, and I sure don’t know how to get it out after all the bleeding I did. I can say with reasonable confidence that air in the system wasn’t the problem.

So...to bring this full circle...I’m wondering if 3/16” lines, and/or a 1-1/8” M.C. would shorten up my brake pedal travel and give me better performance with the discs. I will also say that I won’t rule out the master cylinder could be the problem. It’s brand new, but it was a relatively cheap aluminum aftermarket from who-knows-where. It’s definitely possible it’s bad out of the box, but there’s no way for me to know for sure without eliminating other possible causes, and without having another M.C. to compare it to.

Regardless, I still need to decide on a brake line diameter, whether the M.C. is bad or not, and also need to decide on M.C. bore size if I decide to buy a replacement.

One more piece of information...I’ve read that with manual brakes, you should use a 1” M.C. if your pedal ratio is 5 or less, and 1-1/8” if it’s greater than 5. My ratio is 5, so I’m right there on the bubble. (Peddle ratio is just an expression of the pushrod travel distance against the foot pedal travel distance. It’s measured by the height of the pedal from the pivot point straight up to the center of the foot pad, divided by the distance from the pedal pivot point to the pushrod mounting point) I'm assuming this is good information, but I don't know??

So after all that, have at it. Throw your thoughts and experience at me. Thanks in advance for reading this long-winded post.

Ron

1936 Chevy Low Cab
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