hard pedal doesn't stop after front disc swap

I dsiagree with needing a smaller bore size---a power system typically has a larger bore than manual---15/16 to 1 inch for most manual and 1 1/8 or so for power--I would think a power booster on a 15/16 would throw you thru the windshield.

And power disc boosters have more assist than drum boosters... So you can use a master cylinder with a smaller bore to regain some of the assist.... Also realize the power assist linkage reduces the leverage provided by the standard brake pedal.... With lots of measuring & lots of math you can sort it all out.. But at the end of the day a rock hard pedal with a working booster means either there's a mechanical issue or your math is wrong... If you know the mechanical stuff is right the simplest thing to try is a smaller bore M/C...

Thank you with only 15 inches of vacuum there's not a lot of power assist but it is there and naturally I'm aware of the pedal travel being different on a factory power brake car then a factory manual brake car so you're right there's some math to do

Joe66satellite said:

Thank you with only 15 inches of vacuum there's not a lot of power assist but it is there and naturally I'm aware of the pedal travel being different on a factory power brake car then a factory manual brake car so you're right there's some math to do

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15 inches of vacuum is amazing. It is more than enough to provide adequate booster performance.
Check this out...
1970 Charger.
In 2001, I started with 4 wheel drums and added the 11" front discs I pulled from a '75 Dart. The Dart also provided the power booster, master cylinder and proportioning valve. It worked great BUT the 10" rear drums looked silly in the huge 18" wheel so in 2006 I switched to the Dr Diff rear disc kit. Braking didn't improve or degrade, it stopped about the same.
In 2012, I thought the simplicity of a manual master cylinder would be great so I pulled the power stuff off. I had 4 master cylinders here to try. The largest was a 1 1/8". It bled out fine but was horrible. Hard pedal, barely moved and barely slowed the car down. Next up was a 1 1/32". Same thing.
I tried the iron 15/16" unit from the power booster/Master cylinder combo and while it was better than the other 2, it was still crappy. Finally, I installed an aluminum 15/16" unit from Dr Diff. It was about the same as the iron 15/16" unit. I posted about it on a few forums and got into discussions about pedal ratios, caliper pressures, bore sizes, etc. It went nowhere. I put the power stuff back on and it went back to working pretty decent.
In 2014, I realized that I still had a disc/drum proportioning valve in the system. I changed it out for a drum/drum distribution block since 4 wheel disc systems share a feature that 4 wheel drum systems have: Built in proportioning based on wheel cylinder or caliper sizing. Braking improved. I wanted more though....I took a spare brake pedal and drilled a hole 3/8" higher and welded a washer around it. This increased the pedal ratio. Currently, the car stops great and has great feel.....BUT I still like the look and weight savings of an aluminum manual master cylinder. The change from power to manual saves about 15 lbs too.

There have been tons of threads on here about the do's and don'ts of disc brake swaps. Tons of trial and error and spending tons of money trying to solve the issue. IMO, Chrysler solved it for us by using the dual diaphragm booster with disc brakes on these cars. I know first hand since I was one of the guys that tried using the drum brake booster on my 68 Charger. It is human nature to think that a little boost is better than none at all. That was my reasoning. I had 20" of vacuum at idle and was sure I could make the drum booster work. Sent it out to Booster Dewey for rebuild even though it seemed to work. Tried 3 different master cylinders, bled the brakes a dozen times, changed proportioning valves a couple times. I ran out of things to try and got plenty of wrong info from people who think they know what they are talking about. Finally, when speaking with Cass at Dr Diff., I got the correct info. Change out that damn single diaphragm booster to the dual diaphragm booster like Mopar did and your problems with hard pedal and inability to stop the car goes away. For all those naysayers that want to argue the point, I have a rebuilt drum brake booster for them. You can squander all the coin you want trying to come up with a combination that will work. Stupidity is expensive. My advice, Follow Mopar's lead. They ran into the same issue when they started experimenting with disc brakes. Their engineers would not have spent the money on the dual diaphragm if it was not needed.

Sixpactogo said:

There have been tons of threads on here about the do's and don'ts of disc brake swaps. Tons of trial and error and spending tons of money trying to solve the issue. IMO, Chrysler solved it for us by using the dual diaphragm booster with disc brakes on these cars. I know first hand since I was one of the guys that tried using the drum brake booster on my 68 Charger. It is human nature to think that a little boost is better than none at all. That was my reasoning. I had 20" of vacuum at idle and was sure I could make the drum booster work. Sent it out to Booster Dewey for rebuild even though it seemed to work. Tried 3 different master cylinders, bled the brakes a dozen times, changed proportioning valves a couple times. I ran out of things to try and got plenty of wrong info from people who think they know what they are talking about. Finally, when speaking with Cass at Dr Diff., I got the correct info. Change out that damn single diaphragm booster to the dual diaphragm booster like Mopar did and your problems with hard pedal and inability to stop the car goes away. For all those naysayers that want to argue the point, I have a rebuilt drum brake booster for them. You can squander all the coin you want trying to come up with a combination that will work. Stupidity is expensive. My advice, Follow Mopar's lead. They ran into the same issue when they started experimenting with disc brakes. Their engineers would not have spent the money on the dual diaphragm if it was not needed.

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I think you're probably right booster Dewey matter fact I'm sure you are I've just been in denial and I hate to have to crawl under that dashboard to remove that single diaphragm booster but I have no choice if I want it to be right I guess I'll remove the driver's seat, so I can get under there lol

Joe66satellite said:

I think you're probably right booster Dewey matter fact I'm sure you are I've just been in denial and I hate to have to crawl under that dashboard to remove that single diaphragm booster but I have no choice if I want it to be right I guess I'll remove the driver's seat, so I can get under there lol

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Sorry I meant you're right six pack to go now I've got booster Dewey on the brain

I hate being the bearer of bad news but I had to learn that same lesson after the fact. There is so much bad info out there to buy into. If I were you, I would call Cass (Dr. Diff) to get his recommendation for which parts to get. He won't lead you down the wrong path. Don't buy a proportioning valve from Inline tube. If you want a bad one, I have three from them. The guys on here all have good intentions, but nobody knows it all. Me included. Good luck getting it fixed.

1 Wild R/T said:

Are you sure the booster is working? Try this.. Engine off, step on the brake pedal 3-4 times... Then step on it & keep your foot on it with light pressure... Start the engine.. Did the pedal fall slightly? If not the booster is bad... If it did chances are your gonna need a smaller diameter master cylinder to compensate for the booster being designed for drum brakes it doesn't have as much assist as a disc brake booster...

As was mentioned earlier a pressure gauge would be useful in gathering accurate info to diagnose your issue..

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I wanted to thank you for your comments I did your test and the brake booster and the check valve are working so I bought
that aluminum 15/16 bore master cylinder from Dr diff and what a pleasure doing business with Cass it truly is wow and I'm putting it on that drum booster I'll keep you posted thanks for the support

1 Wild R/T said:

And power disc boosters have more assist than drum boosters... So you can use a master cylinder with a smaller bore to regain some of the assist.... Also realize the power assist linkage reduces the leverage provided by the standard brake pedal.... With lots of measuring & lots of math you can sort it all out.. But at the end of the day a rock hard pedal with a working booster means either there's a mechanical issue or your math is wrong... If you know the mechanical stuff is right the simplest thing to try is a smaller bore M/C...

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Oh by the way everybody I've only got 11 inches of vacuum duh