4 wheel disc conversion on 69 Road Runner

[Gldwing57]

I have been reading several post about brake conversions.
I 1st did my fronts from drum to disc and added power brakes as well.
This worked ok, but not as good as I thought although the car stopped pretty good.

Recently added rear disc and that's were problems started. Low floor pedal and not very good breaks at all.
I have read about some stuff from years ago so I was wondering if anything new out there as far as fixes.
I think my problems are from proportion valve since it's the original for drum brakes.
Does anyone have a part # for one that does 4 wheel disc.
I know there could be other issues but thought I'd try that 1st and also see if anyone has any information they may share.

Thanks
Patrick

 

[Gldwing57]

 

[ckessel]

Paging Greg! @Kern Dog

 

[dfrazz]

Add a hydroboost for best brakes. Rear discs are a waste of money IMO since 70% stopping is up front.

 

[70rrclone]

Im using the ssbc in the pic. Hydroboost with Viper calipers on the front, explorer calipers in the rear. Stops great.

 

[Kern Dog]

When it is set up right, 4 wheel disc brake systems can be great. One problem that many people encounter is braking that is not as good as a stock front disc/rear drum system.
Why?
I personally think it is because the system is pieced together with parts that were not designed to work with each other and they are not compatible.
You can often make it work if you somehow manage to stumble upon a combination where everything is close enough to work.
More specifically, 4 wheel drum or disc systems work best when the sizing of the components match up from the front to the rear. In a drum system, they always seem to have larger wheel cylinders and wider drums up front compared to the rear. For disc systems, the caliper size or surface area is often double at the front than the rear.
Andy Finkbeiner wrote that 4 wheel disc systems have a 2 to 1 bias where the front is responsible for twice the braking as the rear. 66% front, 33% rear. The caliper sizing needs to reflect this. If the front caliper has 6" of surface area, the rear needs to be around 3". The front and rear rotors can be similar size but the clamping force provided by the caliper needs to be double at the front.
For years I had a 4 wheel disc system that was put together with mismatched components. It worked but wasn't great. Today it feels great. I have a stock reproduction brake booster, 13" front, 11.7" rear "Cobra" disc brakes sold by Dr Diff, a 15/16" master cylinder and NO proportioning valve. ONLY a disc drum system uses a proportioning valve because that valve is needed to balance the very different needs of a system that has front disc brakes and rear drums. 4 wheel drum and 4 wheel disc systems may use a distribution block where the brake lines merge but that block has no proportioning function in it. The "proportioning" in a 4 wheel drum or disc system is achieved by the wheel cylinder or caliper sizing.
Cheers!

 

[59pwrwgn]

What did you use for the front brakes and booster. The factory adds a bell crank to the linkage which I think increases the pedal ratio. I read somewhere that manual was 6 to 1 and power was 4 to 1.
When I built my truck years ago, I added a booster and modified the rod from the pedal for it to work. The truck seemed to stop fine, but drums are self energizing. I switched to disc up front and and kept the same master by removing the residual valve. Well the pedal went to the floor and the truck barely stopped. God was I disappointed . After a while I drilled a new hole lower on the pedal to change the ratio. That one change gave me a good hard pedal up high and locked the wheels up. I don't know if it was the booster, master or both that needed more travel to make it work.

 

[Gldwing57]

When it is set up right, 4 wheel disc brake systems can be great. One problem that many people encounter is braking that is not as good as a stock front disc/rear drum system.
Why?
I personally think it is because the system is pieced together with parts that were not designed to work with each other and they are not compatible.
You can often make it work if you somehow manage to stumble upon a combination where everything is close enough to work.
More specifically, 4 wheel drum or disc systems work best when the sizing of the components match up from the front to the rear. In a drum system, they always seem to have larger wheel cylinders and wider drums up front compared to the rear. For disc systems, the caliper size or surface area is often double at the front than the rear.
Andy Finkbeiner wrote that 4 wheel disc systems have a 2 to 1 bias where the front is responsible for twice the braking as the rear. 66% front, 33% rear. The caliper sizing needs to reflect this. If the front caliper has 6" of surface area, the rear needs to be around 3". The front and rear rotors can be similar size but the clamping force provided by the caliper needs to be double at the front.
For years I had a 4 wheel disc system that was put together with mismatched components. It worked but wasn't great. Today it feels great. I have a stock reproduction brake booster, 13" front, 11.7" rear "Cobra" disc brakes sold by Dr Diff, a 15/16" master cylinder and NO proportioning valve. ONLY a disc drum system uses a proportioning valve because that valve is needed to balance the very different needs of a system that has front disc brakes and rear drums. 4 wheel drum and 4 wheel disc systems may use a distribution block where the brake lines merge but that block has no proportioning function in it. The "proportioning" in a 4 wheel drum or disc system is achieved by the wheel cylinder or caliper sizing.
Cheers!

 

[Gldwing57]

Thanks for response. I called a tech from Right Stuff and it seems the guy who installed didn't adjust the rears properly.
He wouldn't sell me an adjustable adjustable proportioning valve until I checked this.
Plus they send me video on how to do it right.
So hopefully that will help.

I sure the guy you mentioned on 67 and 33 is correct. I've always heard 60 40 .

Again I appreciate your input.

 

[Gldwing57]

What did you use for the front brakes and booster. The factory adds a bell crank to the linkage which I think increases the pedal ratio. I read somewhere that manual was 6 to 1 and power was 4 to 1.
When I built my truck years ago, I added a booster and modified the rod from the pedal for it to work. The truck seemed to stop fine, but drums are self energizing. I switched to disc up front and and kept the same master by removing the residual valve. Well the pedal went to the floor and the truck barely stopped. God was I disappointed . After a while I drilled a new hole lower on the pedal to change the ratio. That one change gave me a good hard pedal up high and locked the wheels up. I don't know if it was the booster, master or both that needed more travel to make it work.

I bought the system from summit for power brake conversion. But I agree that could be part of problem, plus the rears are not adjusted correctly .
Thank you for your response.

 

[MoparLeo]

There is no brake adjustment for rear disc rakes. Except a proportioning valve.
With rear drum brakes you can raise the pedal by simply adjusting the shoes to have a closer fit to the drums.
You are suffering the same fate as 90% of the people who do dis brake conversions using non factory parts.
Most of these kits are using GM hydraulics like the calipers.
Rear disc is just eye candy unless you are living in the mountains and like to stop quickly going downhill.
Kern Dog pretty much hits it on the head.
Frankenstein brake systems are "No bueno "

 

[Kern Dog]

Rick Ehrenberg was one to dismiss the advantages of rear disc brakes for awhile. He isn’t the absolute guru on everything but he does make sense about most things.
What he did come to think is that they can be easier to modulate up to the point of lock up. Drums do not work in linear manner, sometimes they will lock up with only slightly more foot pressure than you used at the last stop. Discs ARE linear and easier to feel a lock up condition coming on, giving you the chance to avoid it.
He embraced retrofit of OEM rear disc brakes into older cars because the big factories had a lot more engineering and development into them than small independent brake manufacturers.
It is tempting to go for what is cheap and readily available, places like Summit with free shipping sure do sweeten the deal. Sometimes you get lucky and it works.
Years back a “frugal” guy I knew bought a Master Power rear disc kit with GM calipers. He never could get it to work right. The bleeder screws were in a position where once the calipers were in place, he couldn’t get all the air out. He had to remove the calipers and have his kid hold them in his hands with the bleeder screws at 12:00 with a block of wood wedged in to simulate the rotor. Come on, man. THAT is a product they sold to the public? Really? Yeah.
I started off my rear disc swap entirely for appearance but over time, I finally figured out how to get them to be an advantage.
It only took 16 years.