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Members with 4 wheel disc systems.....Can you get the tires to skid?

Kern Dog

Life is full of turns. Build your car to handle.
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I am having a hard time making sense of my brakes. I've tried multiple combinations and no matter what I do or try, I can't get them to skid on dry pavement.
I'm not a trained mechanic but I do have years of experience with these machines. One method that I use is the process of elimination. If the system has faults and you change enough parts, eventually you will find the problem. This is not easy on the wallet but sometimes it is the only course of action you have to work with.

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This car stops well but it isn't awe inspiring. I never feel like I need to lift from the brake pedal because I am about to skid or stopping faster than I want. I have good parts in the car but something just isn't right.
Originally, it was a 4 wheel 10" drum system. You all know how those perform.
My first change was an A body power booster and 11" front discs. It stopped well and never had me feeling like I was scared to drive it fast.
Later, I upsized to the Cordoba 11.75" front rotors.
In 2006 I pulled the rear drums and installed the Dr Diff 11.7" rear disc brakes. From this point forward, the car got faster but the braking stayed the same.
Last year when I had the engine out, I decided to upgrade the brakes. I installed the Dr Diff 13" front kit and went down the rabbit hole in a failed Hydroboost swap. That resulted in a complete failure either due to a faulty HB unit or my impatience in the bleeding procedure. Regardless, the HB came back out and I tried a manual 1 1/8" master cylinder. TERRIBLE. Hard pedal with horrible stopping force. I tried a 15/16" MC. Better, but still not nearly good enough to be content with it.
I put the A body booster back in along with a vacuum pump and tank and that is where I am today.
It stops good enough but I'm still wondering why it won't skid. To me, it seems like if it can't skid, then the system isn't at it's potential.
I bought a brake caliper gauge....

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I only measured the rear and it registered 1200 psi.
I'm open to any and all suggestions except swapping drums back on the rear. There is something wrong with the system and I just am not seeing it.
Can you skid on dry pavement?
 
Wish I could help you right now KD but my 72 is sleeping for the winter. I run the factory 11.75 front disc and 97 Jeep GC rear disc. Standard MC for my car. I have an adjustable prop valve going to the rears. When I set it up I used a wet parking lot and adjusted the bias so the fronts locked just before the rears. I have not done a hard panic stop on dry pavement but maybe I should. Pedal feels good.
Are you using an adjustable prop valve?
 
No.
I use a drum-drum distribution block that has no proportioning.
Of all the combinations that I have tried, there are a few things that have not changed. I wonder if those remaining things are where the problems are.
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It crossed my mind that maybe this valve is blocking or reducing pressure or volume to the brakes but when I tested the right rear caliper, it showed 1200 psi. Maybe I should take the car out to a parking lot and see if the parking brake can lock the tires. That might tell me something.
 
The only car I had that had aftermarket four wheel disc's stopped really well, but was a complete mess, (and ultimately a disaster). I could lock the fronts, with stock tiny factory Girling disc's, but the JFZ's on the back wouldn't lock up the14x32s. The balance fr/rr was pretty good (by accident).
 
Well, crap....if you're locking up tires that huge, I'd really be impressed.
 
I am using the stock distribution block for front disc and drum rear. When I did my brake upgrade for the front I used the Mopar Action disc o tech article. I remember reading that some disc/drum master cylinders have a restrictor and it should be removed. It is in between the rear reservoir and the rear brake line. Just a thought.
 
My understanding is that the drum-drum blocks have NO proportioning. Those 4 wheel drum systems rely on wheel cylinder sizing to determine proportioning.

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The 73-76 A bodies with front disc-rear drum used a block with proportioning built into it. Some call it the "Texas" valve due to it's shape.

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This unit is somewhat complex. It is a fixed design that limits pressure to the rear to try to prevent lockup. The drums take less pressure to work due to their "self energizing" feature.
I had one of these in the car with 4 wheel discs.
I changed to the drum-drum distribution block and it did improve braking a slight amount.

I've tried manual master cylinders of different sizes, I've tried the power booster with and without a vacuum pump. I changed the front brakes to 13" with performance pads. I changed the rear brake pads. Everything has been bedded in.
What has not changed is the rear calipers and the brake lines themselves. The hard lines are in good condition. The flex lines are new.
I have considered running the lines without the distribution block to see if that helps.
 
I have been kind of following along on this thread and I may be just repeating something already posted. You said you’re currently using a drums only distribution block. Have you tried an adjustable proportioning valve for the rear discs ? Please disregard this if you have done that already.

Our 64 Sport Fury has 4 wheel power disc with only the adjustable prop valve on the rear brakes and will stop on a dime without problems or lock up the front and then rears if you want to. Adjustments were done on a damp parking lot.
 
I'll be watching this thread. I too went with the drum-drum distribution block because I understood it was just a fancy splitter that I could hook up to the low pressure brake light switch.
Tapped a roll control in between the manual MS and front brakes. Adjustable prop valve further back on the frame rail connector. Going with 13" Baers all the way around. It hasn't been pressurized yet but I'm eager to see how it pumps up. Brake issues suck. Good luck Kern, sorry I can't help.

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I have 11 inch wilwood disc brakes up front and 10 inch drums in rear. Disc/drum master cylinder, no prop valve or distribution block. And I've done a few panic stops, never slid once.
 
I agree, locking them up takes braking beyond the point of being fully effective.
My point is that if I can't get them to lock up, I am not reaching their potential.
No, I do not have any proportioning valve in the system. I figured that if I were getting rear wheel lockup, I'd add one then but that has yet to happen.
 
Here are a few combinations that I've tried:
11" front disc 2.6" caliper, 10" rear drum. 1975 A body brake booster. Disc/drum proportioning valve.
12" front rotor, 2.6" caliper, 10" drum. 1975 A body brake booster. Disc/drum proportioning valve.
12" front rotor, 2.6" caliper, 11.7" Dr Diff rear discs with 1.5" single piston caliper. 1975 A body brake booster. Disc/drum proportioning valve.
12" front rotor, 2.75" caliper, carbon metallic pads. 11.7" rootr with 1.5" single piston caliper. 1975 A body brake booster. Disc/drum proportioning valve.
Same as above without booster but with 15/16" iron, 15/16" aluminum, 1 1/16" and 1 1/8" manual master cylinder. Disc/drum proportioning valve.
12" front rotor, 2.75" caliper, 11.7" rear rotor, 1.5" single piston caliper, drum drum distribution block, 1975 A body brake booster, modified pedal for increased leverage.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. Hydroboost unit with 1 1/8" master cylinder.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1 1/8" manual MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 15/16" manual MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 1 1/8" MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 15/16" MC.
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 15/16" MC, electric vacuum pump.
Now the current setup is:
13" front rotor, twin 1.58" piston calipers, 11.7" rear rotor, single 1.5" caliper, drum-drum distribution block. 1975 A body booster, 15/16" MC, electric vacuum pump and storage tank.

Yeah....I have tried LOTS of combinations.
 
From the "glaringly obvious" department:
1. Disc brakes were originally thought by engineers to be preferable to drums because the design
inherently resists locking up better than drums.
2. Modern tire compounds, especially those of some performance brands, are formulated for maximum
grip - i.e., resistance to losing contact with the road surface.
I'd imagine the same qualities that make them so darn good at not sliding going around a corner also
makes them darn good at not sliding under braking, eh?
 
Did you switch to a manual pedal when you've tried no booster? The pivot point is different.
Definitely not great, but I need 2 feet on the pedal with my fury and manual disc/drum combo to lock them up
 
I see a lot of folks recommending a rear adj prop valve, which has nothing to do with what he's experiencing. The adj prop valve is designed to restrict pressure to the rear brakes to keep them from locking up, he can't get them to lock up! So putting a restrictor in the system will do absolutely nothing but make it worse.

KD, I am hesitant usually to give my expertise as it is the internet, as facts have nothing to do with it.
You need to get an assistant and utilize the pressure gauge at all 4 corners and compare them. Jam on the brake pedal and have the assistant record the highest pressure reading. Report back with all 4. In a hydraulic system, the pressures should be the same throughout, but you have 2 separate systems here. I am curious to see what the pressures are doing since your back tires are not locking up.
Another variable is the large wheels and tires you have, the further the tire surface is from the center rotational point is a longer lever arm requiring more force to change its speed.
Use the tool you have and let's get this worked out.

Edit, Ed beat me to it
 
The car had manual brakes when I bought it.
From what I can tell, there is no difference between the manual brake pedal and the power. The power assisted systems used a reduction linkage that fitted under the dash. This booster is from a 1975 Dart. Those systems use reduction linkage attached to the booster itself on the engine side.

"The adj prop valve is designed to restrict pressure to the rear brakes to keep them from locking up, he can't get them to lock up! So putting a restrictor in the system will do absolutely nothing but make it worse."

That is my point as well.
 
I will throw in a suggestion along with the pressure test and that you take out the drum brake splitter and get a direct reading from the master to each corner. Then you have a good idea of what the system is doing without any other variables in the line. I am probably just repeating something already posted but any brake issue is a safety issue in my mind. Please disregard if this has been discussed previously.

Just for information we did some testing with the Fury and the adjustable valve really makes everything work together. But that’s a different set up so it’s not applicable here.
 
This doesn't help the dilemma, but some info on braking. When a tire is skidding, it is sliding on melted rubber, greasy, which is not slow. When going through EVOC (Emergency Vehicle Operations Course) and Bondurant n Bragg Smith school of driving, the most effective braking is "threshold" braking. At the point of impending lockup but not locked. Threshold braking is shorter than ABS, ABS just allows you to steer in panic brake situations.
 
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