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

[Kern Dog]

I can't. I never have.


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 this out.

[b5blue]

How big are your tires? 

[Kern Dog]

That is an interesting point.
275-40-18 up front, 295-35-18 rear with 200 treadwear. They are quite sticky for cornering.

[Kern Dog]

Some ideas that I am considering:
Gutting the distribution block.
Omitting the distribution block and running the lines from the Master Cylinder directly to the wheels.
Testing the rear brakes by seeing if the parking brake is enough to skid the tires.
Checking the pressure at each caliper.

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.

[John_Kunkel]

A tool that I've found useful for such problems is a gauge set that shows the actual clamping pressure at the caliper. 

https://www.toolsource.com/general-service-tools-c-1321_79_84/brake-pad-apply-pressure-test-kit-007884-p-95168.html?sourceid=bingps

[Kern Dog]

That is interesting. I wonder how that compares to a gauge that measures pressure at the bleeder. 

[Kern Dog]

I do wonder if I am chasing something that isn't there or does not matter.
If the brakes are working well enough to stop the car safely and in a reasonable distance, maybe that is enough.
I had the front end aligned and since then, the wheel stays steady enough to stab the brakes and get the car hauled down pretty good. Before, it would wander and pull so I lifted off the pedal to straighten it out.
I have considered the switch to a dual diaphragm booster, one that is OEM for disc/drum 66-70 B bodies. This should (in theory) provide more force than the 1975 A body unit I currently have.

[b5blue]

That's what I put on so my daughter could drive the car, works great but you'll need vacuum. 

[Kern Dog]

Thanks, I am currently using a vacuum pump that pulls 22 inches of vacuum along with a storage tank.

[Derwud]

With Manual Brakes, barely.. With Hydroboost, WHOA, she brakes like a horny man passing a Ho House.

[Kern Dog]

I wish that I could have made the hydroboost unit work in mine.

[Kern Dog]

Hey there,
Just figured I'd make an update. I'm still dealing with braking that should be better than it is.
I still have the same setup as I had when I posted last time. I have new pads front and rear and while they do grip well, I still can't skid.
I can't fit wheels smaller than 16" in diameter up front because of the rotor size or I'd just swap on some narrower tires to see if it is the width and grip of my 18" tires that is preventing me from skidding.
Again, it isn't as if I want to flat spot the tires. It is that I feel that if I can't get them to skid, I just have no idea of the actual brake force I'm getting. My goal is to have enough force to skid but then to brake just shy of the point of skidding. To me, that is maximizing the combination.
I still have intentions of swapping in the 66-70 B body dual diaphragm booster but life has gotten in the way the past few months.
Thanks...

[Kern Dog]

I just ordered the correct dual diaphragm booster, linkage and firewall plate.
Maybe I'll set aside some time to do this swap in the next couple weeks. I'll have to make new brake lines since this unit sits lower and further back compared to the A body unit that I've been using.

[LaOtto70Charger]

When I first got my ssbc setup in with the single diaphragm booster my brakes would lock up. The booster failed and brakes went really bad. Converted to manual and can't lock them up. I have a dual master to try but haven't switched it over as I will need to redo the brake lines also.

[Kern Dog]

In 2012, I tried switching to manual brakes for the first time. It was horrible. Nothing I did made the car feel right.
I tried 3 or 4 different sized master cylinders and NONE of them gave me confidence. All gave a really hard pedal with what felt like 1/3 the amount of braking action that I wanted.
I've been all over the map with this car. ALL sorts of combinations.
A smart man looks for common denominators when trying to pinpoint a problem.
I've used "seasoned" proportioning valves and distribution blocks but always used new fluid, new brake hoses, different pads, calipers, etc.
This A body booster setup pushes 1250 PSI to the calipers but the gauge peaks then falls fast as if the booster does not hold pressure in the lines as it should. Maybe the booster has a small hole in the diaphragm?

[LaOtto70Charger]

A hole or bleed would make sense. With the drum brakes my pedal always needed pumped to not go almost the the floor if not used in a couple miles. With the manual and disc's it no longer does that but pedal effort is high and still lower than it feels like it should be.

[Kern Dog]

I've had disc/drum cars that stop better. My 67 Dart with 73+ discs is one. The car is lighter and simpler but with narrower tires, smaller brakes and a manual MC, the damn thing just hunkers down and stops great.
My other car, Jigsaw has 12" front rotors, 2.75" single piston calipers, 10" rear drums and a 15/16" master cylinder. It stops different than the red car. Pedal travel is longer but when it grabs, it really works. It might be hard to determine which car brakes better.
I have higher expectations with this car though. It is capable of higher speeds and better handling so I want the brakes to be on par with the rest of the car.
I am considering plumbing the master cylinder straight to the front and rear lines with no distribution block. All the blocks I have here are originals from 70-76 year model cars. Who the heck knows what is inside of them. I took one apart and removed the shuttle valve but after some consideration, it seemed likely that a failure in one end of the car would have the effect of a single pot master cylinder....effectively making the entire system fail. If I plumbed the master cylinder directly to front and rear, at least I'd still have the separate reservoirs in the MC to serve as a backup.

[Kern Dog]

I ordered and received a 1966-1970 Dual diaphragm brake booster from Dr Diff. I got it with the firewall stiffener plate and the under dash reduction linkage. I'm squeezed pretty tight with other stuff to do but I hope to get to this project soon.

[Kern Dog]

Life keeps getting in the way here.
I'll bet that this is a familiar thing for you too, huh?
I'm probably no different than you guys....I love my cars but there are often more pressing matters to attend to.
This house has needed some work to get it ready to sell so the hobbies have been delayed a bit.
Summer is the best time here to do almost everything due to longer daylight hours, warmer weather, etc.
The cold and overcast of Winter kills the motivation for me. Spring-Summer-Fall are the times to get things done.

[Kern Dog]

Ah, screw it. I'm going to step away from house stuff to work on the cars a bit.
On Sunday, I was out banging gears and tossed the P/S belt. I got another (and a spare) and I'm back in shape again. next up, the brake booster!
I have a 15/16" master cylinder in the car now. I also have a barely used 1 1/8" unit that I bought for the failed Hydroboost experiment. If this was a factory style front disc/rear drum, I could just copy what the factory did but.....This is "uncharted territory". The story that I hear is that the smaller bores provide greater pressure but with a longer pedal stroke. Larger bores provide less pressure but the stroke is shorter.
I'm going to start with the 15/16" MC I have now. My thoughts are that IF this new booster provides more assist, each fraction of movement of the pedal may have greater effect. If the pedal feels too touchy, the larger bore MC may be a better choice.

[b5blue]

You have changed the total "moving volume" of the system. I don't know how you can translate the math. I'd start in the middle. 

[Kern Dog]

You have changed the total "moving volume" of the system. I don't know how you can translate the math. I'd start in the middle. 

I'm guessing that is a way of stating that there is no clear answer....Arriving at the best combination may be a matter of trying different things.

[Kern Dog]

I started the swap.
The A body booster and linkage is out. The 66-70 B booster and linkage are in but not completely bolted in.
The back side of the booster has a "snout" that the main pushrod comes from. The housing of that is wider than the hole punched in the firewall. I had test fitted and was unable to get the nuts started on the bolts because the booster wouldn't sit flush against the firewall.
I had to take a die grinder and shave off some material and remount the booster.
The nuts and bolts are not easy to reach but with the steering wheel off, I can get to it. Part of the problem is visibility. MY shop feels like a friggin dungeon from the Mount of Monte Cristo. Too damn dark! My one test light is a POS....it has the switch on the side where if you breath hard enough, it shuts off the damn thing. Screw it...I tossed it out. I'll buy 2 or 3 that are built right. It sucks to keep having to put the light under the hood, then under the dash then under the hood again.

[Kern Dog]

Almost done. Like it is with upgrades like this, sometimes one thing affects others so I end up making more changes than I expected.

All the hardware is in place. I first tried bolting up the existing master cylinder. Inside the car, I started it up and the brake pedal went near the floor and stayed there. I reached down, pulled it up and pressed it again. Same thing.
I thought that maybe the booster rod wasn't making contact with the master cylinder piston since the booster is a stock type 4 bolt, my MC is a 2 bolt and I have an adapter in between. My thinking was that maybe I had to go back to a standard 4 bolt master cylinder.
I ordered a 1 1/32" aluminum 4 bolt master cylinder from Force 10 brakes. Who knows when it will get here.
In the meantime, I bolted up a barely used 15/16" iron master cylinder and was ready to test it. Again, the pedal went almost to the floor and stayed there.
It turns out that the power brake linkage on these cars must need some slight amount of slack in the linkage do it can swivel and move. I didn't know this and Dr Diff includes NO instructions with this stuff.
Once I backed off a nut, the pedal swung freely.
Test drive....It now skids on dirt but not really much on pavement. I was only able to get the right front to barely skid about 15 mph as I slowed down from about 60
The great news is that it does feel much better than before. The harder I press the pedal, the harder it stops. Before, it felt like anything over a half effort didn't make a difference. Press  half as hard as I could or with all my strength and it stopped the same.
Not anymore. Under hard braking, it feels fantastic. VERY confidence inspiring.
Score!

[Kern Dog]

I'm not real happy about how the brake pedal sits lower than the clutch pedal. The change to the stock B body booster resulted in the pedal sitting lower than the clutch pedal by maybe 1 1/4". It also made it so my brake light switch has to be adjusted way back to the end of it's range of adjustment.
I'm thinking of modifying the brake pedal pushrod, in effect extending it to raise the height of the pedal.