Technical Brake pedal ratio issues

If your existing brakes have a hard pedal but not enough braking action, you need a smaller Master. This will make the pedal travel more as it is pushing less fluid but with a bigger force to the wheels thus generating more pressure on the brake shoes/pads. Would feel softer but will brake a bit better. The critical point is reached when the pedal is almost touching the floor before the wheels lock up. If you don´t like the further travel of the pedal or the still not satisfying brakes, fit a booster or better/bigger brakes.

 

You might try the 2 3/8 calipers that pprather has suggested, I would think that would help some. Even though a smaller caliper is theoretically not as efficient as a larger caliper, it should help with applying pressure.
If you have thought about going to the trouble of a booster you should also consider a hydro boost system. I installed one on my off topic car and it works great. They require a smaller space in regards to diameter but the length is about the same. There will be some hoses that need to be routed from the power steering pump to the booster but that shouldn't be too hard to address. Hope you can get it resolved, they are much more enjoyable to drive when they provide a secure means of stopping.

 

I was reading that chart the same way at first, but they are talking about a power brake boosted system with that chart. I did measure my pedal and it is a 6:1 ratio.

 

So I finally did the math, using the Speedway article linked above.

Using a 15/16" master cylinder, to match the rear wheel cylinders, the master cylinder area is .69 square inches.

The 2 3/8" alternate brake rotors have an area of 4.45 square inches.

4.45÷.69= 6.45:1 ideal pedal ratio.

If all else is correct within the brake system, the smaller caliper would seem to work closely with the manual brakes and 6:1 pedal linkage in this rod.

Please recheck my math.

 

If finding brake pads for the calipers you have is getting difficult, and those calipers may be causing an issue with the master cylinder bore size and pedal effort, maybe its time to reconsider upgrading what is needed to install more modern calipers?

1968 was pretty early in disc brake production line usage, a lot of stuff has changed since then. Maybe something more modern can be adapted to your existing spindles that may well solve the entire problem. I understand it may well be costly, but better and easier functioning brakes would be worth the expense from my perspective.

Maybe Wilwood has a system to cure all your problems without a booster, and you may well be able to use your current brake pedal. Most of the current oval track cars in the country use their braking systems and I have seen a lot of their systems on street cars. Inquiring as to what they might suggest might be the least expensive way to have great brakes for a long time to come.

 

Wilwood 4 piston fronts with no booster works just find for me on a 56. Way better than 4 drum and I used wider than original rear drums.

 

Two observations:

1. Because you are after the ratio of the cross-sectional areas of the master cylinder and the wheel cylinder, the actual areas become unimportant. The math becomes a lot simpler if you simply leave out π and 0.5², seeing as both appear on both sides of the ratio and therefore respectively cancel each other out. Simply comparing [master cylinder diameter]² to the sum of all the [wheel cylinder diameter]²s will get you the same ratio.

2. Absolute line pressure isn't important in itself. Rather, I'd expect that sound engineering would suggest the lowest line pressure at which the right ratio of areas will apply. That means beginning with the wheel cylinders and calculating the master cylinder diameter from that. You'd want the biggest combination of wheel cylinders which would suit the application, as that would give the lowest line pressure for any given braking force.

 

I had a similar problem on my car, the gold Barracuda convertible and I can tell you how I solved it but it wasn’t easy.

My car originally came with manual drum brakes and they worked well. That system used a straight pushrod. I switched it out to the factory power brake booster setup using all original parts.

The factory set up used what they call a pivot that is essentially a levered contraption and it does two things: It reduces the amount of force you are able to put on the booster so it’s not over boosted but the other thing is does is reduce the total pedal stroke. By reducing pedal stroke, the pedal can be lower to the floor and still fully function.

Sales literature actually points out this change claiming that the change in the leverage point by using a pivot brings the pedal much lower and allows for a shorter stroke thereby making it more comfortable for the driver. My manual pedal was very high and compared to my other Mopars, they were designed to be high. The new boosted pedal position is half of the old and it felt VERY strange to me. Every time I braked, I worried the pedal would bottom out but it never did.

I spoke to many owners of cars like mine with factory power brakes and they all confirmed that my set up was correct. I have a four speed and when you compare the brake pedal to the clutch, it’s so low that it looks like something isn’t right. But it is.

Never being one to leave well enough alone, I took out the pedal to pivot pushrod and custom made my own. The original is 2 3/8” so I added 1/4” of length. The odd leveraged pivot meant the extra 1/4” actually raised my pedal 2 inches. I love it now and it feels more natural. The space in my car with the pivot in place is extremely tight so there was no way to insert an aftermarket adjustable pushrod.

If you end up doing something like this, be very mindful of changing any leverage points if you have a similar pivot piece like mine. I only lengthened a rod to bring the pedal higher. In the future, I may engineer my own custom pivot to change the leverage points for a slightly softer pedal but if I do, I need to be careful I don’t get too much pedal stroke and damage the booster.

 

That Speedway chart is referring to a power brake system that is described in the article.

 

I think we are muddying the water between pedal ratio and master cylinder ratio which will multiply together to give the total (mechanical and hydraulic) leverage ratio.

https://www.joesracing.com/master-cylinder-math/#:~:text=A 1” master cylinder has,785 or 127.4 PSI.

One thing I don't see us discussing is the overall stroke of the master cylinder.

Sometimes these are different between a manual vs power setup in an OEM application. 1" and 1.25" stroke lengths come in to mind as being pretty common.

You need to be able to bottom out the cylinder before the pedal hits the floor. Especially if you want you want redundancy in case of a line failure.

Sometimes a factory manual brake ratio (say 6:1) won't be able to get a newer 1.25 stroke master to bottom out. Sometimes you end up having to give up some mechanical advantage at the pedal to get the stroke, then making up for the loss hydraulically making a lower line pressure do the work needed.

 

\

This must be the same setup that the Cordoba pedal I used had.

Gary