Pedal ratio question - GM A body

[68Malibu383]

Hey guys,

I have a '68 Chevelle with the Kore3 C5 front brakes and factory drums in the rear (plan to install the Kore3 C5 rear discs when time/money permit). I currently have the factory 10.75" booster and a 1.125 bore master cylinder. Engine generates about 11" of vacuum at idle. Have poor performance now, especially in parking lots - hard pedal at low speed and low brake function. Tobin recommended a 15/16 bore MC and I think I'll get the '77 Malibu unit.

Regarding the pedal ratio, I am using the factory lower hole on the pedal and the ratio is approximately 3.3. If I could move the clevis to the upper hole (factory used for manual brakes), the ratio would be closer to 6. However, the rod hits the top of the booster where it extends into the firewall before the clevis lines up with the upper brake pedal hole. I could drill a hole just slightly lower in the pedal arm but am concerned about the angle of the rod - will not even be close to center at the back of the booster hole where it comes through the firewall. I'm concerned about running the rod at an angle that is so far off from factory/being centered in the hole at the back of the booster assembly.

Should I be concerned about that angle and what have others done to safely get a better pedal ratio?

[Apogee]

I remember recommending that you have at least 14+ inches Hg at idle...but seriously, a cam swap and/or auxiliary vacuum pump could improve your vacuum situation. IIRC, other recommendations included a larger dual-9" diaphragm booster to make the most of the vacuum available and a smaller bore master cylinder to better match the rest of your system.

Running increased rod angularity at the booster can potentially damage the booster, so changing the pedal ratio more than a fraction would be my last choice for things to look at. You could effectively do the same by just further reducing the bore size on the MC, but at about 7/8" you're where we would put most manual C5/C6 brake systems. After that, all you have left to play with is pad compound, which can be significant.

Tobin
KORE3

[andrewb70]

Get rid of the booster, run the MC that Tobin recommended, move the rod up for a greater pedal ratio and call it a day.

Andrew

[Arctic_Ragtop]

I just did exactly what Andrew recommends above. I completed a disc brake conversion with the new disc brake master and 9" booster but did not like the look if it or the angle that it sat on using the lower hole on pedal for power brakes. I purchased a Wilwood Master, got rid of the booster and moved the clevis to the top hole, used for manual brakes. Make sure you get a 7/8" bore to maintain the proper brake force to wheels when using manual brakes.

[68Malibu383]

After reading tons of posts earlier, I found one guy with the same brakes and booster and he was running the '77 Malibu MC but he did not mention his pedal ratio. Was hoping to keep the booster so I would have not to redo my lines at the MC. Maybe the easiest and most consistent pedal feel would be to go with manual. Just concerned with high pedal effort and long travel.

[Apogee]

Manual brakes = higher effort and longer pedal travel versus properly set up power brakes, however they can be made to work very, very well. With that said, what you have now is in improperly set up power brake application with several strikes against it already. Everything in your current system is erring towards a low brake torque scenario...low vacuum, small-ish booster, oversized master cylinder, etc. Considering what it's going to cost you to make vacuum brakes work like they should (vacuum pump, MC, accumulator, etc), manual brakes could be a much cheaper and simpler solution if you wanted to try it first IMO.

Tobin
KORE3

[68Malibu383]

Thanks Tobin, I think I'll go that route. Can you/someone recommend an affordable MC? Would that '77 Malibu 15/16 unit be acceptable?

[Apogee]

There are plenty of people running the '77 Malibu 15/16" bore unit in manual installations with good results. The difference between that and a 7/8" unit is about 12%, and we typically run one or the other with manual C5/C6 installations depending on a variety of factors. If you decide the effort is too high, you can always change over to a higher CoF pad compound like the Hawk HP+, EBC Yellowstuff, Carbotech AX6, etc., assuming you're not already running something like that.

Tobin
KORE3