Who has used a 1" bore master with power brakes?

[ZEKE68]

Looking for input for using a 1" bore master cylinder instead of the usual 1 1/8" bore master cylinder for my power brake setup. Is petal travel excessive? do you get more brake force? This is with a disc/drum system.

[ZEKE68]

ttt

[Bigdav160]

In theory it should increase the force at the expense of pedal travel

[68Stepbed]

What's your intended purpose of doing this? Are you wanting better brakes?

[ZEKE68]

I am running a fairly stout cam, and am not sure I will have enough vacuum. I thought that if I had plenty of travel the 1" bore would give me more pressure than the 1 1/8" bore. Just wondering if anyone has actually tried it.

[ZEKE68]

I am running a fairly stout cam, and am not sure I will have enough vacuum. I thought that if I had plenty of travel the 1" bore would give me more pressure than the 1 1/8" bore. Just wondering if anyone has actually tried it.

[Rickysnickers]

I think you should have 12" to 18" of vacuum to run a vacuum booster, if memory serves correctly. If you have a pretty radical cam, you may want to go to a hydroboost system. There's a fellow on the 60s' part of the site with a year old hydrobooster for sale. It may be worth a look.

I have a 1" bore with a vacuum booster. It's all that's been on the truck, so I don't have anything to compare it to, sorry. It works fine though.

[jocko]

It's a good question Zeke, I'd be interested to hear if anyone has experience doing this also. But, unfortunately, while it might help a little - I don't think you're going to solve the lack of vac for your booster issue by the 1/8 smaller bore mc. It will probably still feel like non-boosted braking. But maybe someone has tried it and can give you an idea on whether there is any noticeable improvement.

[Boog]

I know there is science at work here but how does a 1/8" smaller bore master make for better brake pressure when the brake lines are not changed also?

[ZEKE68]

With hydraulics its a piston pushing on a fluid. Assume you have a 1 square inch piston pushing on a fluid like brake fluid. To build 1000 pounds per square inch of pressure in the brake lines with a 1" square inch piston takes 1000 pounds force applied to the rod pushing on the master cylinder. If you had a piston that was 1 and 1/8 square inches it would require 1250 pounds force on the master cylinder rod to achieve 1000 pounds per square inch pressure in the brake lines. Its surface area working on the fluid, and fluid working against the surface area of the piston. Which is why I thought that as long as total travel was not a problem, that I would get more pressure, or have less pedal effort with a 1" bore master cylinder. You use leg power and the lever advantage of the pedal linkage to get the 1000 pounds force. I am trying to see if anyone has already done this to tell me if the pedal travel is a problem.

[mike16]

It does not work like that...

fred Puhn's book "How to make your car handle" explains the relationship between a MC diameter and the wheel cylinder/ caliper set up.

there is also another good book on it and I'll find the details so you can seek it out as well.

buying these books on amazon will save you time money and frustration

the 1 inch diameter MC is for a disc/ drum set up. the 1 and 1/8th dia. mc is for disc/disc set ups.

its not about pressure so much as sufficient volume to push enough fluid to activate the front calipers.

cam vs vacuum is a whole nother problem.

If your going with a vacuum booster and a big cam then you will have problems regardless what mc you use.

but to help you I can suggest that the mid 60's corvette had a mc for non assisted disc/disc set up and the mid 70's caddy eldorado had a vacuum assisted disc/ disc set up.

MC's for vacuum assisted disc/drum set ups are quite common but for a non assisted set up early 70's firebirds had the correct MC.

Again... your problem is vacuum power assist with a big cam not trying to choose an mc with greater volume. Too much mc volume will make the brakes very touchy and essentially uncontrollable. maybe a hydraulically power assisted set up to cercumvent the side effects of your cam choice. Never buy a rebuilt MC. always new.

[ZEKE68]

CCP, LMC, and most other venders who sell parts for our trucks sell 1 1/8" bore master cylinders with their power brake systems used for disc/drum.

[jocko]

Good article on this subj here Zeke: https://www.joesracing.com/rt-4172-m...nder-math.html

Although smaller bore indeed equates to higher pressure, the same amount of movement of the pedal will move less fluid, so you will indeed need to increase pedal throw to actuate the brakes the same as you would have with the larger bore mc. How much, not sure. Some math in the article, but it does a good job of explaining the background. Like I mentioned above, I think the real issue that needs solving is the lack of vacuum - I don't think you can achieve what you're hoping for by bore size alone. You would likely have to change pedal ratio also, in the direction that provides less pedal swing - but then you'd have to use more leg muscle because you'd be losing leverage with the ratio change.

[mike16]

vendors sell what customers demand. that does not mean its right or correct.

most people who put the wrong master cylinder on their vehicle compensate for that mistake by installing an adjustable perportioning valve as well. they either dont drive the truck often enough to be troubled by it or simply become resolved to accept the problem as a side effect of driving an old truck. Neither ought to be the case.

the wrong diameter master cylinder will not create booster vacuum lost due to a cam selection. Otherwise good advice but no mention of what to do when you have no vacuum to assist the booster in stopping the vehicle. That advice is found , as a potential warning, in cam catologs

in the article mentioned above, written by a vendor by the way , it states "consult your brake company engineer". I have my company brake engineer on speed dial. I also have Freds Book.

[Steeveedee]

Going from a 1.125" bore to a 1" bore gives a ratio of 1.26:1. So, you will have to push the pedal ~26% further with a 1" bore. My pedal with power disc brakes moves maybe 1.5" on a hard stop. So, if I put a 1" bore master on it, the travel would be 1.26 *1.5" = 1.9". .4" isn't a big deal. What kind of movement are you getting?

[kwmech]

Didn't some of the later model GM trucks have an electric vacuum pump you could install into the system to the booster to supplement

[Boog]

My daughter's 03 Tahoe has that. I thought it was odd to see that on a stock production vehicle which should have plenty of vacuum.

[mike16]

diesle's dont have vacuum, thats why they have hydro boost