Need help with master cylinder theory/operation, please.

[bobcowan]

Originally Posted by YerDugliness

I've tried your link multiple times, also copied the URL and pasted it into my browser window, can't get it to connect...even added an "l" at the end believing it was an html attachment..sure would like to read it, though, Bob!

I checked out that web site before I posted it. But when I look for it now, the entire HotRodHeaven web site appears to be non-existant. Too bad, as that was some of the best information on brakes I'v come across in a long time.

Originally Posted by YerDugliness

I see the difference between the one I have and the Corvette type, the 'Vette type has two equal sized reservoirs in the master cylinder while mine has a large reservoir closest to the firewall and a smaller reservoir at the front of the master cylinder. Since this is a manual brake system, it makes sense to me that the reservoirs (and, hopefully by extension, the volume of fluid moved when the brakes are pressed) would need to be equal.

Many stock MC's have a bigger resevoir and bore on one end than the other. Those are generally designed for cars with disc front and drum rear brakes. Drum brakes require less volume.

Originally Posted by YerDugliness

I don't think there's any way to adjust the length of travel of the piston. Since I don't know much about brake theory, I'm at a loss for what to do now. Logic tells me that in order to more effectively clamp the disc pads against the disc, more volume of fluid needs to be moved when the brake pedal is pressed. If that is right, I have found a 1 1/8" bore 'Vette style MC at Speedway Motors, see this:

Not exactly. What you need is more pressure, and the same volume.

A bigger piston bore will move more volume. BUT, it will require more pedal pressure to create the same pressure. And that pressure will be created in a shorter piston stroke. Anything more than 1" in a manual system will create a very hard pedal, and a short stroke.

Originally Posted by YerDugliness

However, if a smaller bore is better (keeping in mind that there is no way for me to adjust the "leverage" by increasing the stroke of the piston), Speedway also offers this 1" bore model:

A smaller bore will work better for you. Changing the stroke of the piston will have nothing to do with pedel effort. You can change the leverage by changing the pedal ratio. More on that later.

Originally Posted by YerDugliness

I think the MC needs to be replaced, regardless....

I agree. I think it should be replaced with a dual MC like I pictured above. If I thought there was anything better on the market, I'd be using it.

Originally Posted by YerDugliness

Either way, a proportioning valve will be sourced and installed into the brake line leading to the rear of the vehicle....
Dugly

I would not ( and do not) use a "proportioning valve". What most people call a proportioning valve is really a pressure reduction valve. It doesn't really proportion anything.

Stock cars come from the factory with a combination valve. Part of it's function is to help balance the brakes. They're very carefully designed to work in a variety of situations. AFAIK, there are off the shelf combination valves that will work in a car like this.

PEDAL RATIO: One of the big points from the Hot Rod Heaven web site was the pedal ratio. That appears to be the #1 reason for a hard pedal and poor braking. For a manual system, you want to get at least 6:1, and preferably 7-7.5:1. Anything less will require a large amount of pedal pressure to produce enough line pressure to stop the car.

Measure the distance from the brake pedal pivot point and the center of the pad, where your foot goes. Now measure the distance from the pedal pivot point to the push rod attachment point. Divide one into the other, and that's your ratio.

The other issue is the arc that the brake pedal and push rod move through. The push rod is a straight fore and aft movement. But the brake pedal moves in an arc - and so does the push rod attachment point.

Look at your pedal from the side. If the pedal is straight up and down, that means your rod attachment point is probably at the 6 o'clock position at rest. As you move the pedal forward, the push rod end moves in an upward arc, with less forward motion the further you depress the pedal.

I put a 7* dog leg in my brake pedal. That keeps the pad where it's supposed to be, but moves the push rod attachment point closer to the 4 o'clock position. My artistic skills are not that good, but this might make it more clear.



Before spending any money on any MC, evaluate your pedal ratio and position. This could be a very simple fix.