Who has Dr. Diff 8 3/4 rear discs? Pt. 2

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plumkrazee70

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I started a thread here, awhile back asking for help with my Dr. Diff rear brakes. I am still having the same problems, despite getting his parking brake cables and a factory parking brake lever, cable and equalizer.

So here is the issue... I have a 70 Dart with Dr. Diff rear disc brake kit with the mustang calipers, they have an integral parking brake (single piston applies the brakes and is used for the e-brake) I purchased his 15/16 master cylinder and his front disc kit along with his parking brake cables.

I re bench bled the master cylinder and the entire brake system, adjusted the parking brake cable, but still no matter what I do, with the car on jack stands in drive, the wheels won't completely stop, not with the parking brake applied or with the brakes applied. I am not sure what I am doing wrong.

Here is a video and also some pictures of my e brake cable routing and setup.

Thanks for any help, this is literally the last piece of the puzzle before I can drive it for the first time in about 8 years.

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Looking through my pics, this pivot pin looks too small and too long. Not sure if this has anything to do with my problem or not. The ebrake is plenty tight as you can see in the video.

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Looking at your pics I thought I’d compare with our Australian version of the Dart (VF & VG Chrysler Valiant Hardtop) at the Handbrake lever end. This may help as I can see its assembled differently from yours, I hope it helps you out.

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I have not read your other posts on this topic so I'm flying a bit blind here....
In 2014, I tried to switch to a manual setup. I have a '70 Charger with 12" Cordoba front rotors, 2.75 single piston calipers, the Dr Diff 10.7" rear disc kit and.......A '75 Dart power booster and 15/16" master cylinder.
I bought a 15/16" manual master cylinder from Dr Diff. It was terrible! Hard pedal but horribly inadequate braking. I also tried a few other master cylinders, two of them were over 1 inch in size and one was a factory iron 15/16" unit. ALL of them were bad, they all felt firm but ineffective. I gave up and put the power stuff back on.
It turns out, I stupidly had a disc/drum proportioning valve in the car! I switched to a drum/drum distribution block and braking improved. I have not yet tried to go back to manual brakes but the weight savings is tempting.
 
I have a Dr Diff rear disk brake kit with the Mustang calipers on my 68 Barracuda and they work fine. I have the Mopar 73 and up disk sets on the front and am using the 15/16 master cylinder as recommended by Dr Diff.
The 68 Barracuda was a 4 wheel drum car and I maintained the OE distribution block, although the block and all the brake lines, hard and flex are new.
With that said, you mention that neither the e brake or the hydraulic brake will stop the rear disks. That indicates that the rear calipers / brake pads are not extending to grab the disk.
Can you have someone step on the brake pedal let it release and then apply the e brake while you closely watch the rear brake pads and report what you see. Do the pads not move at all or do they just not move enough. You should also contact Dr Diff directly, he is very good on service support. He does post here on FABO occasionally. I believe there is a manual adjustment that can be made on the Mustang calipers to move the pads out. The Dr would know.
 
I have not read your other posts on this topic so I'm flying a bit blind here....
In 2014, I tried to switch to a manual setup. I have a '70 Charger with 12" Cordoba front rotors, 2.75 single piston calipers, the Dr Diff 10.7" rear disc kit and.......A '75 Dart power booster and 15/16" master cylinder.
I bought a 15/16" manual master cylinder from Dr Diff. It was terrible! Hard pedal but horribly inadequate braking. I also tried a few other master cylinders, two of them were over 1 inch in size and one was a factory iron 15/16" unit. ALL of them were bad, they all felt firm but ineffective. I gave up and put the power stuff back on.
It turns out, I stupidly had a disc/drum proportioning valve in the car! I switched to a drum/drum distribution block and braking improved. I have not yet tried to go back to manual brakes but the weight savings is tempting.

I don't believe I changed prop valves, my car was a drum/drum to begin with. But is there a way to tell the difference by looking at them?
 
I have a Dr Diff rear disk brake kit with the Mustang calipers on my 68 Barracuda and they work fine. I have the Mopar 73 and up disk sets on the front and am using the 15/16 master cylinder as recommended by Dr Diff.
The 68 Barracuda was a 4 wheel drum car and I maintained the OE distribution block, although the block and all the brake lines, hard and flex are new.
With that said, you mention that neither the e brake or the hydraulic brake will stop the rear disks. That indicates that the rear calipers / brake pads are not extending to grab the disk.
Can you have someone step on the brake pedal let it release and then apply the e brake while you closely watch the rear brake pads and report what you see. Do the pads not move at all or do they just not move enough. You should also contact Dr Diff directly, he is very good on service support. He does post here on FABO occasionally. I believe there is a manual adjustment that can be made on the Mustang calipers to move the pads out. The Dr would know.

I contacted Dr. Diff just now. Are you suggesting that I look at the pads with the caliper off? I am not sure I could see anything with it stationary.
 
Looking at your pics I thought I’d compare with our Australian version of the Dart (VF & VG Chrysler Valiant Hardtop) at the Handbrake lever end. This may help as I can see its assembled differently from yours, I hope it helps you out.

View attachment 1715622989

Interesting! Your arm is angles the other way! I am not sure mine can turn that way because of the handle. I'll look at that.
 
I contacted Dr. Diff just now. Are you suggesting that I look at the pads with the caliper off? I am not sure I could see anything with it stationary.

No do not apply line pressure to a brake caliper when it is dismounted, that will fully extend the piston and probably pop it out.
yes you can see the pad to the rotor interface, go look.
 
I don't believe I changed prop valves, my car was a drum/drum to begin with. But is there a way to tell the difference by looking at them?

post a photo of your distribution block. Also check to see if there is a secondary block - valve at the distribution block or further down the lines toward the rear brakes. I did not have one on my 68 Barracuda , but I had read that some mopars have a rear brake hold off valve that allows the front brakes to come to full pressure ahead of the rear brakes set starting to engage.
 
I have Dr Diff econo rear discs on both my Mopars. The Mustang calipers adjust by turning in and out the caliper piston. I push back the rubber boot and carefully turn it with channel locks although there is a special tool available as well. Most chain auto parts stores lend it to you with a deposit.

The drum/drum distribution block is the correct part for disc/disc brakes.

Is the rod that goes from your brake pedal into the back of the master cylinder correctly adjusted?

Is the opening on the caliper that your pads fit into evenly centered on your rotor? I have had to use thin grade 8 washers to center mine. I had to bevel the lug nut openings on the rotors to allow the rotor to seat correctly too.

Also, I found it easier to run a couple lug nuts with larger opening washers down on the wheel studs to hold the rotors securely while I inspect everything.
 
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I have Dr Diff econo rear discs on both my Mopars. The Mustang calipers adjust by turning in and out the caliper piston. I push back the rubber boot and carefully turn it with channel locks although there is a special tool available as well. Most chain auto parts stores lend it to you with a deposit.

The drum/drum distribution block is the correct part for disc/disc brakes.

Is the rod that goes from your brake pedal into the back of the master cylinder correctly adjusted?

Is the opening on the caliper that your pads fit into evenly centered on your rotor? I have had to use thin grade 8 washers to center mine. I had to bevel the lug nut openings on the rotors to allow the rotor to seat correctly too.

Also, I found it easier to run a couple lug nuts with larger opening washers down on the wheel studs to hold the rotors securely will I inspect everything.

I have the took you speak of, but I only use it when changing pads. These calipers came loaded with pads. The whole kit went together without a hitch, so I wouldn't guess that anything needs to be modified. I've been doing my tests with the wheels on, I guess I could do it with just the rotor (by using the lug nuts.
 
post a photo of your distribution block. Also check to see if there is a secondary block - valve at the distribution block or further down the lines toward the rear brakes. I did not have one on my 68 Barracuda , but I had read that some mopars have a rear brake hold off valve that allows the front brakes to come to full pressure ahead of the rear brakes set starting to engage.

I will see about a pic. I know for sure there isn't any other block except the one on the differential. I'll post both.
 
First thing I would fix, would be the pivot pin. It probably doesn't have anything to do with your issue, but I don't want to find it was something that I assumed was not the issue. Second. I went with the larger rotors, and calipers. I could not get a pedal. Went to a 1 1/32 master cylinder bore. No issues. It could be you need more volume.
 
First thing I would fix, would be the pivot pin. It probably doesn't have anything to do with your issue, but I don't want to find it was something that I assumed was not the issue. Second. I went with the larger rotors, and calipers. I could not get a pedal. Went to a 1 1/32 master cylinder bore. No issues. It could be you need more volume.

Thanks, I'll see if I can find the correct pivot. Everything that I have read and Dr. Diff also recommend the 15/16 cylinder.
 
I will see about a pic. I know for sure there isn't any other block except the one on the differential. I'll post both.
The distribution block and the T at the rear axel look like the ones that are on my 68 Barracuda, and are not causing brake issues, so your problem is not likely to be there.
I do agree with the comment above that the rear pads / rear caliper need to be adjusted. I also had to use a thin washer set on each side to center the pads-caliper to the rear disk.
 
The distribution block and the T at the rear axel look like the ones that are on my 68 Barracuda, and are not causing brake issues, so your problem is not likely to be there.
I do agree with the comment above that the rear pads / rear caliper need to be adjusted. I also had to use a thin washer set on each side to center the pads-caliper to the rear disk.

. Cass just emailed back and said that the park brake cables are routed wrong and may be kimked. they need to go under the housing. So I'll change that also.

Where did you put the thin washer? I know th kit came with some and I did use them. Was it where they mount to the bracket, effectively bringing the pad closer the the rotor?
 
I had the same like issue until I rebuilt a distribution block that I bought used here, my original was gummed up pretty bad. There's kits to rebuild them. Mine looks just like yours.

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