Members with 4 wheel disc systems: Can you lock your wheels?

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Kern Dog

Build your car to handle.
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I can't. I rarely ever have.
Well, I have locked one or two on wet pavement and the dirt surface in front of my shop. On the road though? Only a few times

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.



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....

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 and I am determined to figure it out.

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I can't. I rarely ever have.
Well, I have locked one or two on wet pavement and the dirt surface in front of my shop. On the road though? Only a few times

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.



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....

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 and I am determined to figure it out.

View attachment 1716045435
You should be able to. I can lock mine up with 4 wheel drums and a single pot master cylinder.
 
Yeah, the car in question is the Charger.
Drum brakes are easy. The self energizing function means that as they start to grab, they exponentially want to grab more. This feature is great for reducing pedal effort but makes it hard to modulate to avoid lock up and loss of control.
Front disc, rear drum works great but you still run into the risk of rear wheel lock up.
I am experiencing the opposite.
I can try to lock the rear wheels using just the parking brake. I can run the brakes hard and take a thermal gun to the rotors to see if they are hot.
I may take the brake pressure gauge to each corner. I may try to bypass the distribution block by running the lines from the master cylinder directly to the 4 corners.
 
I can, I went by what Cass recommended, he said to gut the distribution block.

I couldn’t before otherwise.
 
I can, I went by what Cass recommended, he said to gut the distribution block.

I couldn’t before otherwise.
1 hmmmm.jpeg


I have not had contact with him about this. It is worth a shot. I have several spares that I can pick from to use.
 
I could on my Dart. 73/4 Duster fronts, and Lincoln Versaille (Ford) 9" rear with factory disk. "Just to see" if it would work, I used the stock all drum master. I punctured the residual valves with a small nail, and it uses 1/3 to less than 1/2 the pedal. Lock them right up at 70, I tried it a couple times. My legs (then) were sure not what they once were, and I had no problem.
 
Lack of stopping power can be caused by too many reasons, pad quality, to small of pads to rotor surface. I don't believe you have that problem due to the disc kits you installed, you've tried other size m/cylinders, I would look into more assist to increase the pressure with the larger bore m/cyl. When you went to the hydro booster what kind of pressure pump was recommended or was it a kit and may have needed different pulley sizes to increase or decrease pressure? I'm just throwing ideas out there for you to look into. I've read some of your builds you do great work.
 
The hydroboost was powered by a Saginaw pump. Lots of OEM Chevy trucks used a Saginaw pump with hydroboost so I figured that I'd be okay using it for a vehicle that weighs 3900 lbs rather than 7000.
I never could get the HB to build pressure. Later on I saw that the PS cooler had a kink in the return line so I'm not 100% sure of where the blame lies. I was so frustrated with the effort, I just removed it and abandoned the idea.
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.
 
I found the EBC yellow brake pads grab a lot better than the typical street pads. They work on the street while cold but are good on the track when hot.
 
With the hydraulic boosted assist on my `cuda ragtop......easily. 6-piston front 4-piston rear.

35iy8zm-jpg.jpg
 
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Subscribed. I have Dr. Diff front discs 10.95 discs (73+ Abody) single piston and cobra rear 10.7 discs. I have drum dist. block and 15/16 MC.

I have also felt braking wasn't "great" I've thought about gutting the distribution block.
 
Why on God's Green Earth do you think you need 11.7 in rear rotors? That's DUMB. How big are the pistons on those calipers?

ETA OK I see it now. Yeah, your rear brakes are literally as big as the front.

I'm betting those pistons are also ridiculously huge [confirmed], and you can't build any real system pressure because you're moving so much fluid. Something like 70% of your braking occurs from the front, so your brakes need to be sized accordingly. i.e. 70/30 would be 70 percent on the front, and 30% on the rear.

I'm betting your car would stop harder AND faster with smaller rear brakes. Think about how insanely small a wheel cylinder is compared to a front brake piston. With the setup you have, you're sending half the braking force to the wrong end. I'm amazed that car doesn't swap ends every time you stab the pedal.

One of these days I think you guys will realize that just because Dr. Diff (or anyone else) sells it, doesn't mean Dr. Customer needs to buy it.
 
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Why on God's Green Earth do you think you need 11.7 in rear rotors? That's DUMB. How big are the pistons on those calipers?

ETA OK I see it now. Yeah, your rear brakes are literally as big as the front.

I'm betting those pistons are also ridiculously huge [confirmed], and you can't build any real system pressure because you're moving so much fluid. Something like 70% of your braking occurs from the front, so your brakes need to be sized accordingly. i.e. 70/30 would be 70 percent on the front, and 30% on the rear.

I'm betting your car would stop harder AND faster with smaller rear brakes. Think about how insanely small a wheel cylinder is compared to a front brake piston. With the setup you have, you're sending half the braking force to the wrong end. I'm amazed that car doesn't swap ends every time you stab the pedal.

One of these days I think you guys will realize that just because Dr. Diff (or anyone else) sells it, doesn't mean Dr. Customer needs to buy it.
Sorry it's all relative. Bigger pistons mean a bigger master cylinder, and that is relative to the smaller pistons/ smaller master. A person in theory SHOULD gain more braking effect with larger rotors because the larger rotors have more "leverage" so to say, just like stopping a spinning wheel by grabbing the outer diameter of the tire instead of further to the center.

I have no idea what the problem is. Maybe the boost system is not generating the pressure it should for some reason. On a hydroboost the pedal should be quite "hard" and on a manual system, ROCK hard.
 
Just a random example from my own world............the 4WD converted 01 Dodge RAM Cummins.


The 2WD truck has factory front disc/ rear drum and hydroboost. Pedal was near rock hard and LOTS of power. I once got caught in one of the damned "too short" amber traffic lights on a 55mph highway, towing the trailer and a pickup on board. I nailed the brakes and locked them up on the rear wheel ABS I was PISSED

Now that the truck is converted to 4WD, and completed in the early snow of Thanksgiving, I may not have got the air all out, OR it may be that the thing now has about 4 more feet of brake hose in the system---the "new" 4WD truck pedal is a little bit spongy but it still stops like nobody's business.

ON THE OTHER HAND my stupid "nice" low miles 04 GMC has VERY spongy brakes. 4W disk, vacuum booster with a factory vacuum pump ----- and even though decent brakes for when it's empty, I WOULD NOT want to load that truck up to near gross weight with those brakes. I have replaced the rear brakes (disk) and new front pads, bypassed the 4W ABS module, and bled them TWICE. All told I've run nearly a gallon of fluid through the thing. It is minutely better--BUT STILL SPONGY
 
Brake bias.
Andy Finkbeiner wrote about it. In a passenger car, you want approximately a 2 to 1 bias with the front brakes being responsible for approximately 66% and the rear, 33%.
The current brake setup that I have fits directly into that. Twin 1.58" calipers up front, single 1.5" piston caliper in the back. This exact system was installed in some Ford Mustangs from 1994 to 2001.
 
Why on God's Green Earth do you think you need 11.7 in rear rotors? That's DUMB. How big are the pistons on those calipers?

ETA OK I see it now. Yeah, your rear brakes are literally as big as the front.

I'm betting those pistons are also ridiculously huge [confirmed], and you can't build any real system pressure because you're moving so much fluid. Something like 70% of your braking occurs from the front, so your brakes need to be sized accordingly. i.e. 70/30 would be 70 percent on the front, and 30% on the rear.

I'm betting your car would stop harder AND faster with smaller rear brakes. Think about how insanely small a wheel cylinder is compared to a front brake piston. With the setup you have, you're sending half the braking force to the wrong end. I'm amazed that car doesn't swap ends every time you stab the pedal.

One of these days I think you guys will realize that just because Dr. Diff (or anyone else) sells it, doesn't mean Dr. Customer needs to buy it.
Did you bother to read post #10? Though the 11.7" rear rotor seems large, it's only got a 1.5" piston - compared to a 13" rotor up front with dual 1.58" pistons. I would think the front brakes in that configuration EASILY out-power the rears by a factor of two. The rear rotor size is NOT the issue.
 
Brake bias.
Andy Finkbeiner wrote about it. In a passenger car, you want approximately a 2 to 1 bias with the front brakes being responsible for approximately 66% and the rear, 33%.
The current brake setup that I have fits directly into that. Twin 1.58" calipers up front, single 1.5" piston caliper in the back. This exact system was installed in some Ford Mustangs from 1994 to 2001.


This is a discussion of area related to the piston, not diameter. A is pi*r^2
A of 1.58 =1.96in^2. you have two. Double that.
So, 3.92 square inches per caliper.

A of a 2.75" single piston is 5.93"^2.

If you generate 100psi in the system, for example, you would have 593 pounds force on the rotor with the single 2.75 piston versus only 392 pounds force with the double.

Going to the larger 13" rotor over 11.75 nets you 10% better "leverage" but also increases inertia unless you got it to weigh the same. Remember, this is unsprung mass as well.

I missed that you had double piston calipers but my point remains, those rear brakes are huge and don't need to be, and just because they sell it doesn't mean you need to buy it. Your single piston calipers with the rear brakes you have gets you right near 66/33 distribution in force. Step down the rear pistons either thru diameter or metering or rotor reduction, and I bet you'll see an improvement.
 
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Did you bother to read post #10? Though the 11.7" rear rotor seems large, it's only got a 1.5" piston - compared to a 13" rotor up front with dual 1.58" pistons. I would think the front brakes in that configuration EASILY out-power the rears by a factor of two. The rear rotor size is NOT the issue.
I did. See above.
You want the fronts to out power the rears by more like 70%, and his setup does not do that unless my math is way off
 
This is a discussion of area related to the piston, not diameter. A is pi*r^2
A of 1.58 =1.96in^2. you have two. Double that.
So, 3.92 square inches per caliper.

A of a 2.75" single piston is 5.93"^2.

If you generate 100psi in the system, for example, you would have 593 pounds force on the rotor with the single 2.75 piston versus only 392 pounds force with the double.

Going to the larger 13" rotor over 11.75 nets you 10% better "leverage" but also increases inertia unless you got it to weigh the same. Remember, this is unsprung mass as well.

I missed that you had double piston calipers but my point remains, those rear brakes are huge and don't need to be, and just because they sell it doesn't mean you need to buy it. Your single piston calipers with the rear brakes you have gets you right near 66/33 distribution in force. Step down the rear pistons either thru diameter or metering or rotor reduction, and I bet you'll see an improvement.
Doing the math, the 2.75” calipers resulted in a 3.36 to 1 brake bias. This setup is closer to 2 to 1.
The 13” brakes netted a 7 lb weight reduction on each side!
 
More weight on larger wheels to stop. Look into a Dual Diaphragm Booster from a truck for more assist to increase pressure.
 
I should weigh the wheels to see how they compare to stock ones. I do know that my old spare was 59 lbs. it was a 225-75-15 on a 15 x 6 steel wheel.
 
Have you thought about an adjustable proportioning valve? Not that it matters if you're unable to lock up either side, front/back, but just spit-ballin.
 
Just asking;
how big is your engine? and what cam is in it? and what is her vacuum at 1200 rpm or so?
And what size tires are on the back versus on the front?
Was this car an original 6-cylinder car? I think Six banger cars had different pedal ratios; I think maybe so did DB cars. Not sure which was which.
Does the throttle snap closed to idle at around 750 rpm and does the engine display good engine-braking. What I'm getting at, is the brakes are for stopping the car, Not slowing the engine down.
 
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