Power to manual conversion didn't work before but I'd like to try again.

[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?

 

[Badvert65]

Have you looked into where the brake pedal and m/c rod connect? It is not unusual for the manual brakes to connect closer to the lever pivot(top). This give more leverage for braking. Some brake pedal levers will even have two holes, one for power and one for manual. If you have to, you can even drill a new hole. I have done a few conversions and always connected the m/c rod higher on the manual brakes. Each of my cars uses a 76 Volare manual disk master cylinder (1.03 bore) and I have no issues. One car has 11 disk and 11 drums, the other 10 inch disk and 10 inch drum. No issues with either. I used a C body proportioning valve on the Fury and an A body proportioning valve on the Belvedere. Rear disks are something different of course. Biggest thing to watch out for would be the residual pressure valve (for drums) that can cause disk brakes to drag.
Should be an easy swap, very frustrating when things go awry.

 

[Greg Ducato]

I agree that your problem is most likely pedal ratio, you can plumb a simple adjustable proportioning valve into the rear line.

 

[Kern Dog]

I calculated the manual pedal ratio 2 years ago and it wasn't too different than the A body cars I've owned that stopped great without a booster. The car had 4 wheel drums when new so the pedal is unchanged. It would seem that going back to manual wouldn't be a problem except that a disc system needs more pressure. wouldn't a smaller bore MC give higher pressure?

 

[Kern Dog]

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 and look less cluttered.

 

[dangina]

I'm running 13" disks front, 11.7" rear. 15/16" aluminum MC all from Dr. Diff. No disribution valve. Wilwood prop valve on rear brake line. Used a brake "Tee" for the front brakes. you can go fancy and just buy this all in one which I should have done but just recently found:
http://www.wilwood.com/MasterCylinders/MasterCylinderValves.aspx

part #260-11179

 

[ga66mopar]

All Power brake boosters (disc or drums) on 66-70 B-bodies and later used this linkage to connect the booster to the brake pedal.
The factory power brake bootser on the 70 GTX did not connect directly to the brake pedal.

http://www.ebay.com/itm/BOOSTER-LIN...Parts_Accessories&hash=item3389d541a2&vxp=mtr

 

[Kern Dog]

That linkage piece for the OEM b body brakes: I'm having a hard time understanding how it attaches. I need to see an undisturbed car to get a clear understanding of it.