The Stovebolt Site Forums The Shop Area Driveline Let's talk brakes, lines, and master cylinders..

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

I am confused as to whether you currently still have the Huck brakes on the rear or have already converted to the more modern Colorado drums. I know a lot about Huck brakes and very little about discs but if you still have the Huck brakes at this point your symptoms could be caused by lack of adjustment of the Huck shoes. You must adjust them so that there is slight drag. As long as the drums don't overheat it is Ok not to back them off like the manual suggests. If you have already converted to the Colorado drums ignore my comment.

Kent

I'm wondering why you're choosing to run disc brakes without a power booster. I doubt if I've ever seen a setup like that. Regarding line diameter, the only thing a bigger line accomplishes is more fluid in the line. once the system is bled properly the lines could be an inch in diameter without affecting the ratio of master cylinder displacement VS caliper piston movement. I'd choose a bigger diameter MC piston, and a small power booster that will still fit under the floorboard.
Jerry

Sorry for the confusion. I ran the Huck brakes with the discs for a very short time after I installed the discs. Right now the truck is in quite a few pieces getting ready for a 235/T5/Colorado rear swap. So I'm really asking about what to run with the Colorado rear. I only through in the part about the Huck brakes to get thoughts on my my pedal travel was so long.

The Hucks were adjusted to slight drag. I suppose that's objective and maybe my idea of "slight drag" is too slight, but they are both definitely adjusted with some drag to them.

The short answer to that is I'm using the stock pedal assembly with a bracket I fabricated to fit the Corvette M.C. to it. Due to the size (width) of the new M.C., it's already mounted further rearward than the skinny stock M.C. in order to clear the stock pedal bracket. If I ran a booster, it would push the master back even more mostly (or maybe even completely) under the floor (meaning the permanent nonremovable portion) and would be a pain to check and fill.

I've seen plenty of master cylinders, including the one I bought, advertised as manual disc/drum setups, so I didn't think it was all that unusual??

OK, just to be clear, I'm not asking about master cylinder displacement. Obviously the same volume of fluid will be displaced by the M.C. regardless of line size. But would that fluid have to travel further in a smaller diameter line than it would in a larger line due to less volume per inch of line in the smaller line to accommodate the same fluid displacement as the larger line? If so, then caliper piston travel would also have to be longer.

Again, I'm not a fluid dynamics person, and could be completely off in my thinking. I've seen some articles that confirm my thinking, and some that contradict it. No idea whether the authors know anything either! Obviously somebody has to be wrong.

To add to the info given. The size of the line can be 3/16 or 1/4, whatever you want. Same performance. No worry about "distance" in the case. Our old trucks use 3/16 all around. The master cylinder creates the pressure, It's a closed system. A larger MC piston will have a shorter pedal travel.
If you want to go deep which you don't need to.... There is no conflicting info about this.......but here we go.....

A so called "proper" pedal travel is normally calculated for disc/disc. So you will not get an accurate "proper" pedal travel...AND you are thinking of modifying the MC rod. So your "desired" pedal travel to fit your physiology would be calculated with voodoo math and is on you. I would think more of modifying the arm to move pedal pad, not messing with MC rod stroke. That will get you in all kinds of trouble. There is a specific way to set up the MC push rod and must be followed and left alone. I think you could possibly adjust the pedal stop to give foot room, then adjust MC rod to spec. as long as there is enough stroke for MC.

But anyway..... you are shooting for 5:1 to 7:1 pedal ratio for mechanical and 4:1 to 5:1 power assisted.
Pedal ratio is the distance from center of pedal up to pivot point of pedal arm, divided by the distance from pivot point of pedal arm to where MC rod connects to pedal arm.

If you want to know what pedal ratio you need....keeping in mind you only have disc in front....... you calculate the area of the caliper piston diameter and the area of the bore diameter.....and divide the caliper area by the MC bore area.

I.E. 1" dia bore and 2.5" dia piston= 4.91... desired ratio = 4.9:1 You can play with pedal geometry and bore size and etc to reach the foot room and desired numbers.

You forced me or I lost my kiss compass, to make a mountain out of a mole hill and to contribute to over think.

I should have known better than to put all of this stuff into the post. It's causing confusion and focus on the wrong thing. My fault, not the readers' fault. I lost my "kiss compass" too! If overthinking was a profession, I'd be a billionaire!

Anyway, you mention the pedal ratio calculation. I put that in my original post, so I'm well aware of that. I'm right at 5:1. That's built-in, and not changeable with the stock pedals unless I fabricate/modify new pedals, or replace them. Stock pedals are cast iron, so that's not going to happen . I can weld cast iron, but it's not something I've done often and I'm not going to bet my life on the quality of my cast iron weld to not break in a panic stop situation. I'm set on keeping the stock pedal assembly, so I have to work with what I have. The ratio should be good enough for a manual application as per the formula, and of course, since it was manual to begin with.

As for my desire to lower the pedal to be closer to the floor, that's not a priority and I shouldn't have even mentioned it. That just complicated the conversation. I was only using it to point out that I think there is either something wrong (because the old brakes didn't require that much pedal travel) or if there is nothing wrong, is there a way to change this with brake line and M.C. changes. As a side point, I understand what you're saying about shortening the pushrod. That wouldn't change the stroke length that the M.C. piston requires, but it would affect pedal travel since the pushrod mounting point travels in an arc, and not a straight line. But this isn't the focus point of what I'm trying to accomplish.

At this point I think I'm just going to re-plumb the entire system with 3/16 line, and re-use the 1" M.C. If there is no difference, then I'll buy a 1 1/8" M.C. That for sure will move me the direction I want to be. My leg should still be strong enough for the bigger M.C. I still wonder, though, if there is something wrong with my current M.C. It just doesn't seem right that it requires so much pedal travel to reach enough pressure to get good braking, and it doesn't seem firm enough. Even my brake light doesn't active until the pedal is pressed quite a distance. After the extensive bleeding I've done, something just doesn't seem right. Maybe it's just a result of the disc/Huck combo and will be better when the Colorado rear is installed. Who knows...


Since Jerry mentioned my "odd" manual disc/drum setup, I'm wondering if this is really a problem? I don't see how a booster changes anything other than requiring less pedal effort on the driver's part. It won't change pedal travel or fix any of the issues I'm concerned about. It would just be easier on my leg I would think??

Anyway, thanks for engaging in my over-thinking!

I have this for my 53 3100. I haven't tried it.

A booster will make it easier on foot pressure. Nothing else. You do need to wait til you get the rear drums changed to do muvh thinking on this. But as said, shoe brakes need to be pretty close to dragging when set up. Also maybe the proportioning and/or residual valves need adjusted or eliminated.

I assume you have set the MC rod to just 1/16 travel before it starting to push MC piston.
I assume you bench bled the MC before install.
disk/drum is very common.
Never heard of disc/huck but I don't know anything wrong with it.
Be sure pads are really close to rotor also.

After thinking about this some more today, I had to post to acknowledge what Jerry said as being correct. I was too caught up in thinking about what is happening inside the lines, and I should have been only thinking about what is happening at the caliper.

I am correct in that any given volume of fluid will have to travel a further distance in a smaller diameter line than a larger one. The flaw in my thinking, however, was then just assuming that would also translate into the piston having to move a further distance. It does not.

The piston doesn't care what happens inside the line. It only cares how much extra volume is delivered to it. It's movement is only dependent on how much extra volume is delivered to it, and in both 3/16 and 1/4 lines, the volume stays the same and is determined by the master cylinder. The velocity in the smaller line will be higher, but that doesn't matter either.

This does make me wonder why it seems to be a common recommendation to run 3/16" line to the front brakes, and 1/4" to the rear T, then 3/16" from the T to the drums. I've seen this recommended over and over again on different forums. What would be the purpose behind the 1/4" line to the rear T?

What would be the purpose behind the 1/4" line to the rear T?............None. Some vehicles have that. Some have all 1/4. Some have all 3/16. Depends on manufacturer, number of caliper pistons and size, fitting prices and availability.... Speaking of cars and light trucks only.

Not sure if this is applicable, but on AD trucks, there is a distribution block on the rear of the MC, that splits the lines into one 3/16” line that supplies the front drivers side brake and a 1/4” line that supplies a three way block on the passengers side frame, 1/4” in and two 3/16” lines out. One 3/16” line goes to the front passenger side brake and the other supplies the two rear brakes. I suspect the 1/4” line provides more volume, considering it is suppling fluid to three brakes.

I had a horrible time making my front disc conversion with 1966 C10 rear drums work. I think the problem turned out to be that I didn’t use the locking tool for the combination valve while bleeding. Once I finally got the results of that oversight resolved the brakes now work perfectly. The effort is light, and the truck does stop more quickly if I brake more quickly. I had no interest in changing pedal height because it matches that of the clutch pedal.

Robert Crandall -- That's interesting. I didn't have a combination valve, but I did have a proportioning valve. I opened the valve fully (most power to rear brakes) while bleeding. Seemed like the right approach, but there was no specific instructions on bleeding.

I installed a 67 Corvette MC on one of my vehicles las year, converting to power assisted dual system. The MC came with instructions on how to bleed it. I just has to install the two plastic threaded plugs suppied. fill the MC and do two or three easy strokes. The MC was then completely bled and funtional. I also bled one system first with the other blocked at the MC.