Hello everyone,

I am the mechanic that Jim Maxwell is trying to help with this braking problem. I wanted to give you a little update on what is going on with this 7000 series Cobra.

We sold this car on consignment to a local customer. Being what this car was and the fact that is wasn't our's, we never drove the car. After the new customer took the car he complained that the brakes did not stop the car very well. Jim drove the car and agreed that they weren't very good. I put the car on the lift and had someone slowly press the brake pedal until I could no longer turn the rear wheels. Then I checked the front wheels which turned with ease. I took the car out for a drive and tried to lock up the brakes. It took a lot of pedal pressure to stop the car. I could lock up the rear brakes but the fronts didn't seem to help stop the car. This seemed to confirm what I experienced with the car on the lift. It does have Baer calipers, 2 piston front, single piston rear. Front master cylinder is an AFCO 7/8" and the rear is 5/8" Alcon.

I am still trying to resolve the problem. The front master cylinder main seal had a scored place in it and I found a small piece of metal in the cylinder. I decided to replace the whole cylinder instead of rebuilding it and after much bleeding to make sure that I had all the air out, I am still plagued with the same problem.
I could set here and type all night to tell you all the things that I have checked and done. And of course everyone that I have spoken to about this problem has never heard of it and are as befuttled as I am. I haven't given up and will update you soon.

In american cars and trucks that do not have a proportioning valve, the larger master cylinder always goes to the front brakes. That is where the greates amount of braking force is done and is required. Usually about 70% is a ball park figure. On motorcycles it can be as much as 85% bias for the front brakes

Braking
 

Rule of thumb is 75% F and 25%R. The goal is to get the bias at 75% braking power to the front and 25% braking power to the rear.

Interesting previous post regarding the "smaller the hole the greater the pressure" postulation. This is true as can be. If that is the case and if we want more braking effort to the front, then we have to ask ourselves the following. In order to achieve greater braking effort to the front do we need more pressure or more volume?

Does the bore size affect the pressure coming out of the master cylinder? It probably does but I doubt that it affects the volume. If the outlet piping feeding the system is the same size, what is the effect of using a 3/4" as opposed to a 7/8" bore? Will the size of the bore affect the bias as much as it will affect the pressure required to be applied at the pedal?

I would think that an easy solution, allowing us to forego all the mind-bending physics, would be to use two identical master cylinders; equal in reservoir size and bore and adjust the front vs. rear bias using a balance bar pedal setup.

Make it stop on command! That is the goal!

Tony

Update
 

Another thing that must be considered when talking about manual brakes, is the pedal ratio. The goal is to achieve a 5:1 Ratio. If the distance from the pedal pivot point to the end of the pedal arm is 10", then the distance from the pivot point to where the pedal arm meets the master cylinder piston pushrod should be 2". This, I have been told is the optimum point for the greatest braking efficiency. Look to achieve a 5:1 Ratio.

For power brakes the issue is moot as the booster is compensating.

Tony

Tom,

I am sure you checked for a kinked brake line......

Hmmm...

Tony brings up an interesting point...the pedal. Is the front master being activated?

Pat, I was actually going to suggest the very same thing. I have a collapsed front line on one of my sleds, and sadly know the symptoms well--need to fix it actually as I have locked a wheel in front of my house while practicing late braking (something I know you can understand if you are as ill as I am). If it is truly a front/rear bias issue, are there just front/rear feeder lines before they split off to right and left front or rear. Maybe check those? Anyway to test for even front/rear lockup versus just one corner wheel lock before the others?

But you didn't !

I did.

Pat,

You're just quicker than me, hopefully not in all areas. ;)

I went through similar symptoms with my car after purchasing it and couldn't lock up the brakes, even if I tried. Don't know about CSX's, but my Unique has a mechanical brake bias adjustment on the pedal. Mine was set about 50/50, but after a 2 minute adjustment to 75/25, the fronts lock up when I want them too, and braking action is quite good.

If the master cylinder for the front brakes is seized up or doesn't move freely, then the rears will lock up.

How's that, David? :D

Chris,

You stated, "The front circuit cylinder should be on the bottom of the pedal box as it gets actuated first, and the rear cylinder on top. If they are installed wrong, then the rear brakes will be actuated before the front."

Is Shelby still using the basic original design of the pedal box with a bias bar placed vertically between the master cylinders?

Thank you,

David:) :) :)

Kris.

David,

I have never looked inside the pedal box of an original.

You're welcome.

Kris

I edited my previous statement.

Check the masters and see if it has the bias adjuster. It might be sa simple adjustment. I would put new street pads on the front. If the front pads are for racing, they will take forever to seat in and forever to heat up and work. I had this same problem on mine and it was pad related. Next, I would try a 3/4 front master. A smaller master will give you more mechanical leverage, but you will have more pedal travel. Scott

Kris,

Sorry for the misspelling of your name--thanks Turk.

When I looked at Turk's car, the pedal box bias bar assembly was essentially identical to an original car.

Also, did I understand the mechanic correctly, the Baer brakes Shelby uses in the rear are single piston--or slider calipers?

Many thanks,

David:) :) :)

Guys, there is a LOT more that goes into even master cylinder size than this. The master cylinder size in relation to bias, stopping power etc is only but one of MANY factors. By the by, the larger the master cylinder the LESS hydrualic pressure it creates for a given input. Smaller master cylinder is MORE pressure.

I am still blown away by no adjustment of any kind being available. Much less SAI lack of involvement.

Jim and/or tj1949, I have a spreadsheet where you can enter all the components and you will know wether or not the components you have are in the ball bark. Drop me an email and I will send it to you and help you work through the process. It will also give you numbers so that you can do some quick testing on the car and verify that things are working as they should.

Rick

David,

The rear calipers are a single piston design from PBR.

Rick,

I don't think the componets are mis-matched. As I stated earlier, the entire system was setup by Baer to work specifically on the intended application. I also don't understand your comment about SAI's lack of involvement. Do you assume that because nobody is posting here that the problem is not being addressed?

I'm finished.

One more comment. If there is no adjuster, both masters have push rods that thread into the lever attached to the brake pedal. You could back the front master push rod out a few threads and turn the rear master push rod in a few threads to get some more front bias and see if that helps. I have a hi-tech with the pbr rears and wilwood 4 piston fronts. I would bet the wilwoods are close to the same volume as the baer fronts. My front master is 3/4 and the rear is 5/8. I think the jbl cobra is 3/4 front and 5/8 rear also. Good luck, Scott.

I kind of doubt that there is a component mismatch as well. But it seems that they have gone threw it fairly well and still no answer. So at this point, and not being there, grasping at straws.

As for SAI involvement. Unless I missed something I have seen no mention of any help from SAI. One of there cars with 500 miles should be taken care of them by them. Not some third party mechanic. Again, maybe I missed something. I would be very suprised if SAI was helping through this forum. I suppose my assumption is that since they have turned to us for help, they are not getting the help they need from SAI.

Rick