Power brakes VS manual.

[Kern Dog]

 
My 70 Charger had 4 wheel 10" manual drum brakes until 2001. I bought the car in 2000 and within a year I swapped in some 11" discs from a 75 Dart along with the MC and booster. I also used the OEM proportioning valve from the Dart.
In 2006 I pulled the rear drums and installed a rear disc kit offered by Dr Diff. It consist of 10.7" Toyota vented rotors, Ford Cobra calipers with a 1.75 bore and a custom mounting bracket.
At the time I was running a '509 cam in my 493 and the 5-6" of idle vacuum made slow speed stops difficult. I thought that a switch to manual brakes would be an improvement. I figured that the stopping power would be equal to or greater than the power setup ever was.
Wrong.
I tried 4 master cylinders with different bore sizes. The biggest was a 1 1/4". I also tried a 1" and 2 different 15/16" units. Every one of them gave pretty much the same results: FIRM pedal but terrible braking performance. It was actually unsafe to drive over 20 mph and I am not an over cautious guy. I brake tested each MC and only once was I able to get the tires to skid. I weigh 195 and I gave the pedal everything I had.
I was baffled as to why the car wouldn't stop. I tried using different proportioning valves. I tried drum brake units, disc brake units....Each time I drove the car I got the same thing. I asked around to see if anyone had any insight. I didn't get much help then even though there were some suggestions. Finally I did what I really hate: I admitted defeat and put the power booster back on.
I want to try again. With my history still clear in my mind, what other changes should I consider to make this work? The smallest MC I tried was a 15/16" one. I tried both drum/drum combo valves and disc/drum prop valve. Neither were modified. Yesterday I read that Dr Diff suggests to use a gutted drum/drum brake combination valve. Could this have made the difference???

[ChargerST]

You need to do some calculation to figure out which diameter your MC needs to be. variables in a manual system are: piston diameter, MC diameter and pedal ratio.

Have a look at this: https://www.markwilliams.com/braketech.aspx

[Kern Dog]

I will check out that site. thanks.

Looking in my parts shed, I found a power booster that came off of a friends 70 GTX. It even has the firewall stiffening plate. One thing occured to me:
The A body booster I have has a standoff bracket to move the booster up and forward. This was probably done only because of clearance issues in the tighter A body engine bay. The linkage from the brake pedal to the booster includes a few levers. I wonder if the pedal ratio is reduced or increased as a result of it. I might measure a spare A body booster tomorrow to satisfy my curiousity.
The Booster from the GTX looks like it just bolts through the stiffening plate through the firewall. There is no additional linkage. The brake pushrod of the GTX unit connects from the brake pedal directly to the booster. This tells me that the pedal ratio of power and manual is the same on my car. The pedal has only 1 hole.
I am curious if the GTX booster would have a better pedal ratio. If so, this could be a solution. The firewall mounting position looks like it would provide 5" of additional clearance. It would also be tucked back

[ChargerST]

I will check out that site. thanks.

Looking in my parts shed, I found a power booster that came off of a friends 70 GTX. It even has the firewall stiffening plate. One thing occured to me:
The A body booster I have has a standoff bracket to move the booster up and forward. This was probably done only because of clearance issues in the tighter A body engine bay. The linkage from the brake pedal to the booster includes a few levers. I wonder if the pedal ratio is reduced or increased as a result of it. I might measure a spare A body booster tomorrow to satisfy my curiousity.
The Booster from the GTX looks like it just bolts through the stiffening plate through the firewall. There is no additional linkage. The brake pushrod of the GTX unit connects from the brake pedal directly to the booster. This tells me that the pedal ratio of power and manual is the same on my car. The pedal has only 1 hole.
I am curious if the GTX booster would have a better pedal ratio. If so, this could be a solution. The firewall mounting position looks like it would provide 5" of additional clearance. It would also be tucked back

The pedal ratio is different on power brake cars compared to manual cars - even though they use the same hole in the brake pedal. This is achieved with a special linkage which actively reduces the pedal ratio from 6:1 (manual cars) to 3:1 (pb cars) - the rod hanging down in the picture connects to the brake pedal whereas on manual cars the rod from the master cylinder directly connects to the brake pedal!. reducing the pedal ratio also reduces the pedal travel. A booster than adds a fixed amount of brake force to the force that is applied by your foot thus making it easier to brake.

If you directly connect the brake booster rod to the pedal it would be far too easy to brake - you would bang your head on the dash every time you step on the brake. In case of low vacuum (.509 cam) a brake booster is pretty much useless and increasing the pedal ratio helps.

Here's a picture of a booster with the connected linkage:

[71charger_fan]

I have the same brakes you do, including the Dr. Diff kit. I'm using the Raybestos MC36406 master cylinder that Dr. Diff recommended and it stops great with manual brakes.