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 with the system and I just am not seeing it.
Can you skid on dry pavement?

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.

Since the front brakes provide the majority of the braking you need those pressure numbers too.

Let's just focus on what you have right now, the other combos will just confuse the issue. Get the PSI readings at ALL FOUR corners, then let's go from there.

I have Brembo 4 piston calipers on 13” rotors in front and Dr. Diff 11.7” in back. 15/16” master cylinder with no booster and I’m reusing the 4 wheel drum distribution block.

For the first 10 years? with this setup I never locked em up but I never tried (Edit-> except autocross.) The pedal effort is pretty high so I figured that was why I never locked up. Middle of this summer I put new carbon metallic pads on, first auto cross afterwards I burned two flat spots onto my front right tire

just out of curiosity, what is your brake pedal ratio ?

Pedal ratio is an interesting topic.
There is a lot of incorrect info out there so I'll do my best to be clear on how I arrived at my number.
When I measured my pedal, I checked the distance from the center of the pad to the end (from which it swings), then I measured from the pushrod hole to the end. I divided the two. I came up with approx 7 to 1.
This is when I'm directly attached to a manual master cylinder.
The A body power brake booster has reduction linkage built into it that I have not calculated.
I have some things to test just to gather more information. I want to measure caliper pressure at all four corners.
I am considering trying a gutted distribution block to see if it allows more volume and pressure to flow. I'll measure caliper pressure afterwards for comparison purposes.
I intend to weigh my wheels and tires as well as some stock steel wheels like what I have on my other Charger.
The parking brake ....I want to see if it generates enough force to lock the rear wheels from 20 mph, give or take. (Parking lot, of course)

It was a 4 wheel manual drum brake car. I installed a rebuilt 4 speed pedal assembly, should be the factory pedal ratio

I had the car aligned on Tuesday. I asked the man to aim for the maximum caster that he could get, .75 NEG camber and 1/8" toe IN.
Would you believe......
He was able to get 5 degrees of caster on the left, 5.5 on the right?
Yeah....stock UCAs with Moog offset bushings. That is it. I've seen others struggle to get 2 degrees of caster even with these bushings. I don't know of anyone else that has been able to get this much caster without using aftermarket control arms. If so, I'd love to hear about it.
With the front end tracking straight, hard stops seem pretty damn good. It still won't skid but it stops straight and quick.
I may just settle for what I have now and maybe revisit this topic later if need be.

Can you lock up the brakes on wet pavement? If so, which wheel or wheels lock first?

I try to avoid driving in the rain but when I first got the car up and running, I did run it up and down my neighborhood. Some roads had wet patches. Only the front wheels have skidded and even then, it wasn't much.
I slightly skidded the RF on a right turn. The LF skidded when I passed through a damp section.
It actually stops well but won't skid.
With the 13" front brakes, I can't just slip on some of my 15" wheels to test the brakes. I need a 16" minimum to clear the calipers. 15s clear the rear brakes.
I did think of slipping on some skinny tires. I have a pair of 16x7 steelies but they are modified police car wheels. I am not sure that they would clear.

Looks like Mustang calipers on the rear. Make sure they are adjusted for minimum pad clearance.

I can't identify the front calipers, is there an OEM application?

I'd look up master cylinder bores for the donor vehicles and just as important the stroke of the cylinder.

You may be running out of pedal travel before reaching high enough pressure. And 1200 sounds low to me.

This right here, plus you have to regularly use the parking brake to keep them adjusted. Any slop in the rear pad to rotor clearance will affect over all braking performance.

I've had multiple issues with the Cobra calipers on the rear of my Coronet. I've had to replace the calipers a couple of times when they fail to work properly. Not sure why I've had such bad luck with them, they seem to work fine in an OEM application. But when they get out of adjustment they will really reduce braking ability.

i was also under the impression you need to use the parking brake to keep the calipers adjusted when using ford calipers on the rear.
and as was already mentioned, check to be sure you are getting the full travel needed with your brake pedal, and it's not hitting something under the dash causing it to stop before it reaches the travel necessary to fully use the master cylinder stroke.

Just to point out, there's a few things that might prevent fully travel. Linkage binding or hitting something, the booster might not have correct travel, and even the master cylinder can bottom out before reaching high enough pressure.

Disregarding pedal pressure, has this been run as a manual setup? Were the brakes able to lock up then?

Note, the Mustang calipers are different from the Explorer ones. No parking brake in those.

Interesting, my Mustang had parking brakes in the rear caliper, guess it depends on exactly what year Mustang we are talking about. Blowing up the OP's picture, it sure looks like there is a spring there for the parking brake lever. In fact, those calipers look just like the ones on my 96, which most assuredly did have the parking brake built into the caliper.

I don't see any actuating cable for it though,

These are the Dr Diff 11.7 brakes with the 1994-2004 Mustang Cobra calipers.

Those do have the built in parking brake that needs to be used to maintain proper rear brake adjustment and proper brake system function.

I have heard that. I have a 5 speed manual so I do use the parking brake every time I park the car.

To clarify, Mustang calipers have the parking brake. Explorer do not, crown vic do not, F150 do not.

are the explorer, f-150, and crown vic parking brakes internal to the rotor ?

Ah, my misunderstanding. No sure why you decided to throw that tangent into the conversation though.

On a related note, the Dr Diff 10.75" rear disc setup ... which allows the factory and most preferred tapered axle bearings (and using whatever Mustang calipers)... does NOT allow clearance of the factory sway bar end links... therefore... you have to change the sway bar to an under-axle-design. That was a big surprise... I discovered it the hard way.. Cass confirmed (I wish it was indicated in his description of the setup). I had to remove my custom 1" factory-shaped upper sway bar... purchased a Helwig 7/8" (three point adjustable) under axle sway bar... a bit of a hassle to install ... but satisfying when all done (plus the purchase price.. fortunately, Summit had it in-stock and I recvd it in a cpl days).

Using the Cordoba 11.75" front rotors and factory calipers, along with the Dr Diff 10.75" rear discs, I cannot lockup the rears... only the fronts under severe braking... so far, relatively satisfied. I installed an adjustable proportioning valve... I leave it completely open... no bias needed towards front or rear... I could've probably not even bothered with that install.

Forgot to mention I have a proportioning valve to the rear brakes but I have never adjusted it, it’s wide open also. Pad material has had the greatest effect for me

what size master cylinder bore does the stock mustang use ? [what year application ?]

I dunno offhand, got rid of all my Mustang stuff years ago, but I do know the Cobra's, which is the source of the calipers, uses hydroboost.

97 SVT Cobra Mustang shows a 1” master cylinder with hydroboost

Well, crap.......That system would produce a LOT more fluid pressure at the calipers.

This is interesting. I somehow missed this when you first posted it.
A former Moparts member that is on FABO and FBBO mentioned that these Ford calipers have some weird quirks. Like what Sniper wrote, the parking brake ties into the caliper performance. I've read that the parking brake needs to be pressed and released several times to take up slack that forms as the pads wear. Failing to do so results in greater pedal travel before the brakes engage.

In addition to all of this, since 2001 I've used this '75 Dart single diaphragm brake booster:

Some people have said that the single diaphragm booster is likely inadequate for a 4 wheel disc system in such a heavy car. I'm just shy of 4000 lbs.
The factory dual diaphragm booster has been suggested.

I had this same concern when I swapped my front brakes to a configuration I made with viper calipers and a 14" rotor. I run the mustang cobra rear discs. After some initial driving, the pedal felt great, but I thought I would have been able to lock them up on demand. After many continuous hard stops, I could feel them grabbing harder once I got some heat in the pads. I run a standard parts store pad, but I'm sure a more performance oriented compound would change braking performance greatly. Try doing repeated hard stops from around 50MPH. I bet they start grabbing more. Just like Mitch, I installed an adjustable prop valve for the rears and its wide open. There's no need for one. FWIW, locking brakes doesn't decrease your stopping distance.

I know that. MY point was that if I'm not able to lock the brakes, I don't know if they are working to their full potential. If I could lock them, I'd know where that point of lock up is and brake up to just below that point.
Think of it like a throttle cable that isn't adjusted to allow full carburetor throttle blade opening. You could be driving around with a maximum of 3/4 throttle and not making the most power available.

I’m working with Mustang Cobra brakes on a Factory Five Cobra. I replaced the front pads (Hawk) because they would dust the rims with very little use. I went to Akebono ceramic low dust, and the pedal effort is much harder. I have tried to lock them up but so far unable…I need to try again. But iam running very sticky track tires. I might buy a new set of pads and hope they improve.

Pad material seems to have a big effect. My Just Suspensions front brake kit shipped with some dusty pads that were borderline scary when cold, and I don’t recall being able to lock em up. Carbon metallic pads installed last summer, less dust, ok when cold, and they do lock up

I dunno, my 96 Mustang had no issues locking up it's 4 wheel disc setup and it uses very similar calipers and such as the Cobra variety.

So it's not caliper or pad issues, it's lack of hydraulic pressure, imo.

A guy that was a respected member of this board, who is now posting at FBBO instead, said that the Mustang rear calipers need to be adjusted periodically. He said that sometimes the caliper pistons don't move out as the pads wear and that slack creates longer pedal travel.
Because of this, I pressed the parking brake pedal almost 20 times yesterday before going for a drive. The pedal travel was the best that it had ever been and I thought that I had stumbled upon the FIX for it all.
No, I did not.
Within a few miles, the pedal travel increased.....then the travel was long enough to trigger the warning light. The front reservoir was almost dry. The rearmost reservoir had about 1/2" of fluid in it.
I haven't checked the level in a few months but I found no leaks anywhere.
I went ahead and set up the Motive self bleeding tank and set to bleed the right rear corner first. I always slip some clear surgical tubing over the bleeder nipple with the other end into a bottle, all to contain the fluid and to minimize the mess. Clear fluid started coming through which was unexpected. I'm using DOT 5 fluid now. I guess when I bled the system last time, I didn't notice and didn't bleed it long enough to get all the DOT 3 out. Mixed fluids can result in a mushy pedal. I was getting inconsistent pedal feel for awhile. One time the pedal would have good feel, other times it might feel mushy. It was not confidence inspiring.
Out on the road, pedal feel is even and consistent now. The pads bite better once they warm up. I still can't get them to skid but they do slow the car down nicely. It is in no way dangerous, just not awesome. I may need to do some stops next to a modern car to see if they are comparable. If I'm on par with the Wife's car, I'll be impressed.

curious, cause your running a similar setup to me, but I couldnt see anywhere if your running a inline brake bias of some sort for the rear tires - do you have one in place?

By "brake bias" do you mean a proportioning valve?

If so, I do not.
In theory, 4 wheel disc systems should have all 4 calipers and rotors sized in a 2 to 1 ratio with the front having twice the braking force as the rear.
My front calipers have 2 pistons, the rear calipers have 1. The front rotors are 13", the rears are 11.7".


Some well intentioned people have suggested the use of a proportioning valve but I don't think that they fully grasp the issue here. A prop valve does not increase pressure, it only reduces it within the line in which it is installed. If I can't skid now, I won't be able to skid with even less pressure to the brakes.
My situation comes down to insufficient force at the calipers. The booster is either too small for the job or it has a leak, not allowing it to reach it's potential.

You keep mentioning booster or some other forms of assistance.

Can you lock them up manually? Maybe get your 6'8" neighbor to drive the car and stomp the pedal.

Assistance or not, if you press the pedal hard enough, it should lock up.

It isn't as if I am a 98 lb weakling. I'm over 200 lbs and in decent shape.
When I state that I can't lock up the brakes, I'm not bullshittin. They will not lock. They wouldn't lock with a manual master cylinder and don't with this A body booster.

what size master cylinder diameter are you using? you need a smaller bore to raise pressure in the lines.

what pedal ratio do you have ?

The master cylinder is a 15/16".
Regarding pedal ratio, my brake pedal actually has a higher ratio than stock. I drilled a hole above the original pushrod hole to increase the leverage. The A body booster and linkage are all on the engine side as opposed to the B body setup where the booster bolts to the firewall and the reduction linkage is under the dash.
The increase in ratio at the pedal is minimized by the reduction linkage but in short, the ratio is still greater than stock, even if only by a small amount.

i believe at some point there was actually a 7/8 bore diameter master cylinder available too. may want to look into swapping to that.

I don't want increased travel.
The A body brake booster is a single diaphragm unit. The B body version is a dual. It provides double the boost. I suspect that the difference in the two will make the biggest improvement.

I went an looked up the booster my old 96 Mustang had, V6 car 4 wheel discs.

8.732" dual diagphram.

When you say " The B body version" what does that mean, specifically. There are many years of "B body" out there.

Here's a link to an excel spreadsheet that will let you play with the booster numbers. It was in Feet's brake thread pinned at the top of this forum.

manual vs vacuum - uhcoog1.xlss the one you want

http://www.3gduster.com/brakes.html

Sorry for the confusion.
This A body booster is a single diaphragm, the 1966-70 B body booster is a same diameter but a dual diaphragm, effectively providing double the boost.

Man oh man I was so excited to see this thread. I started reading eagerly hoping that you found the fix for this…. Sadly we are in the same boat…. Well sort of. I have a 68 barracuda with 4 wheel disc brakes from wilwood. Their master cylinder, proportioning valve, and calipers etc…. Complete wilwood system meant for my car. Or so they claimed.
The car doesn’t stop well. I cannot lock the tires up either. I re-drilled my brake pedal to create a better pedal ratio and no change in braking performance. Wilwood was no help in trouble shooting. I went from a 15/16 manual master to a 7/8 manual master both from wilwood. I have 1100 psi at the calipers with 100lbs of pressure on the pedal.
The pedal feels great, not hard, not soggy or soft, but car just don’t stop like it should. I am waiting on a set of race pads from a friend to try out.
I have replaced every hard and soft line in the car, all couplings and tees have been replaced and no change, I even replaced my proportioning valve hoping maybe I had a defective one?!?!?

I feel you on this. My car has tripled in power but the brakes have not come along with the rest of the car. It makes me hesitant to take it on the road or even the track.

I think a booster would help but it won’t fit with the big block!!!!

I have been sidetracked with home repairs lately so I haven't been able to follow through with this.
My intention is to switch to a 66-70 B body dual diaphragm brake booster and see how it feels with that. I just don't have the time right now.

Mark Williams recommends a MINIMUM of 1200psi for a 4 wheel disc setup. 1000 is for disc/drum

https://www.markwilliams.com/brake-system-pressure.html

that's what i was thinking, 1200 or more.
then, what kind of bias do you have for front and rear tire sizes ? [or not]
when [if ?] i get my hotrod going, i have tiny diameter front tires and large by huge rear tires, so this is going to be a challenge for me to get it right, master/caliper/pedal ratio wise.

I registered 1200-1250 momentarily but the pressure dropped off with the pedal being held firmly. That shouldn't happen, in my opinion. The pressure should remain steady or you're not keeping consistent pressure to the pads and rotors. Either my booster is leaking or the pump just craps out.

Or no good master

To bad you gave up on the hydro boost. Hydro boost is a great solution. My brakes in a Challenger = Hydroboost, 11.75 dia stock slider calipers front, Ford calipers out back, 4 wheel disk GM prop valve (straight thru). Rear parking brake hooked up or not, either way. Easily skids the tires. And that's with a large bore master cylinder.

I should check the psi at the calipers with hydro boost now. Never did because it solved the problem. I'm sure its way more than 1200. When I checked my manual brakes setup with 7/8 bore it was 700 psi. When i switched back to stock power booster (single but large diameter), engine vacuum was 14 and the psi was 1200 at front caliper.

My opinion about stock booster is you need to get the vacuum up near 19-20 to get impressive brake performance.

The vacuum pump I have puts out 22 inches of vacuum.

Hydroboost = 1800psi., but that's pushing the pedal harder than normal.

I tried a HB system and it was an epic failure. I couldn't get the HB to build pressure and damaged 3 power steering pumps in the process.

What kind of pump are you using? Does it have any shims in it to reduce pressure? I'm asking because I'm thinking about going to a hydroboost setup on my car.

Its the stock power steering pump for a 73 challenger. I have some poorly done video's of the install on my youtube page.

https://youtu.be/xFVBDHaSEtQ

FWIW I believe max tire braking effect is when tire is turning at approx 70% of road speed. Other than maybe F1 skilled/talented drivers that is an unattainable goal for us mere mortals. Even Standard ABS does not achieve that on an instantaneous basis, but might get close on average, in that brake pressure is cycled/pulsed to a large degree. The key is not letting the tire stop or slow long enough to allow the rubber to turn effectively to liquid grease at the road contact point. I am sure there are many variables that effect the ideal limited slippage for any given situation to achieve maximum braking.