Power Disc Brakes not working on hard stop?
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Please let us know what you find! Fixing mine is on my list this winter.
Inertia
Well-Known Member
I thought the hard pedal was something new, after it worked fine for 10 yrs ,
Good luck
Update:
Received the pushrod tool for rod adjustment between Booster and MC - unfortunately the rod was adjusted perfectly so not the issue. I've got a couple emails out to "experts" and will see what they have to say.
Received the pushrod tool for rod adjustment between Booster and MC - unfortunately the rod was adjusted perfectly so not the issue. I've got a couple emails out to "experts" and will see what they have to say.
Dartman,
Read post #14.
Given the accurate & detailed description of the problem you outlined, I am simply amazed at some of the illogical suggestions [ such as p'rod adjustment when you clearly stated that the brakes start applying with 1" of pedal movement ].
Other than a defective booster, there is only one other thing I can think of that could cause this problem: corrosion in the m/c. On light brake application, the m/c seals against a smooth bore & max pressure is developed. On hard braking, seals move a bit further into a corroded area of m/c bore...& fluid leaks past. I think this is unlikely, but it is possible.
Read post #14.
Given the accurate & detailed description of the problem you outlined, I am simply amazed at some of the illogical suggestions [ such as p'rod adjustment when you clearly stated that the brakes start applying with 1" of pedal movement ].
Other than a defective booster, there is only one other thing I can think of that could cause this problem: corrosion in the m/c. On light brake application, the m/c seals against a smooth bore & max pressure is developed. On hard braking, seals move a bit further into a corroded area of m/c bore...& fluid leaks past. I think this is unlikely, but it is possible.
Swinger 340
Well-Known Member
Some good replies to your question, but I would like to add one more. Have you changed out your rear brake line to axle (rubber), if not it can be soft and expanding while putting pressure on them.
Thank you everyone! After reviewing all the responses and evaluating my experiences with 2 systems (SSBC conversion on a 67 RT Coronet (SSBC uses the same setup on B & A bodies) and my 67 Dart (Stock MOPAR setup), I believe I'm experiencing the natural limitations of the smaller rotor and disc brake system. Simply put, its at the end of its braking power. The only way to overcome it is to go to larger calipers and rotors, which would require larger, bolt pattern with larger wheels and tires. I've traded emails with both Dr Diff and SSBC and neither can offer any cures with my current setup. Steps I've taken: Power bled the system, measured the MC pushrod to ensure max fit and travel, checked all rubber lines, checked for vacuum leaks and found none, ensured booster and MC operate as expected. The brakes work great under normal circumstances with good stopping power and easy pedal. When I brake aggressively while going fast I reach a limit where more pressure doesn't provide more braking ( a hard pedal not on the floor). The fact that I have the luxury of testing 2 different cars with the smaller calipers and rotors and they act similarly leads me to believe this is normal. While I was hoping to find a way to improve I discovered this is the best 1960's technology had to offer. I appreciate everyone's contributions.
Inertia
Well-Known Member
Have you tried different pad compositions.
Are they ceramic pads ?
Are the pads glazed ?
Different compositions can have vast differences in stopping power.
blu should be along for the minutiae.
Are they ceramic pads ?
Are the pads glazed ?
Different compositions can have vast differences in stopping power.
blu should be along for the minutiae.
Some ideas:
1) The original KH disc brake pistons can seize in the cylinders as a result of moisture. You might try putting the car on jack stands and seeing if any wheel can be turned by hand or by breaker bar when the pedal is applied.
2) You mentioned a manual proportioning valve. Is that the original valve that is located in the rear brake line, bolted to the frame below the driver? It's shaped like the state of Texas. That's the only prop valve you should have. The original distribution block does not have an internal prop valve. It's only real function is to add the brake warning light, otherwise you could just use a Tee connector at the MC for the front brakes, and run the rear brake line directly from the MC. Some other model mopar distribution blocks do have internal prop valves, so you should make sure you have the correct block. I don't know what the affect of running 2 proportioning valves would be, but it might be a problem.
3) Or, do what I always do: email Dr. Diff.
Inertia
Well-Known Member
Lotta folks don't realize there are proper break-in procedures for diff composites.
On my ssbc setup, the pedal feels like it is hitting a mechanical stop. It doesn‘t seem to have that little bit of sponginess you get when pushing on the pedal harder on normal brakes.
I don't know if you want to hear this, but thats not it.
There is no reason in the world that if the hydraulic pressure was applied to pads and rotors in good condtion, they would not be able to lock up the front wheels. Same brake system (minus the booster) works well on my car and many others. I've locked up race tires (flat spotting is not a good thing!) so the system is capable - something is wrong.
I would investigate the hydraulics, the booster and linkage, as well as the rotor surface and pad lining.
Been through similar and there is no obvious definative answer from the info posted.
Yes, That is the exact feeling I get with both systems (cars). I've traded emails with both Dr Diff and SSBC and neither have any suggestions for me. Are you also running 14 inch wheels with the small bolt pattern?
I am running 15 inch SBP cragars. 195 width tires up front, so I should be able to skid them pretty easy.
Have you tried opening the front and back bleeders to see if the pedal goes to the floor? That might be a way of isolating the problem.
I was going to try that, but I haven’t got to it yet.
Also thought about loosening the master cylinder up about 1/2 inch (or as long as the bolts are & see what that does.
Need to figure out how to divide & conquer….
It certainly isn't that I don't want to hear it - I'm open to any and all suggestions - I simply don't know where to go with it. I'm attaching 2 pictures, 1 is my 67 Dart with stock MOPAR system and the other is a 67 RT I just converted for a buddy. The Dart is 10+ years old and the RT is a brand new SSBC conversion ( kit A156-1 fits both B & A bodies). They both come with a manual proportioning valve that mounts between the stock prop valve and the rear brakes to be adjusted so the rear brakes won't lock up - this is a manual adjustment done once by trial and error until you get it right - they are adjusted correctly. The back wheels won't lockup on a hard stop and allow the system to apply greater pressure to the front brakes. I've checked the Booster to MC rod and both are adjusted correctly per the tool I purchased and listed previously. The Dart has stock rotors and calipers the RT has Drilled and slotted rotors and stock calipers (I believe SSBC uses a 67 Mustang setup - not totally sure). I have good travel on brake pedal and good operation under normal driving. They both hit a hard pedal with what feels like no further pressure on the brakes on a hard stop. This is same as described by "Mike69Cuda". I've emailed Cass at Dr Diff with this information and while he offered a single piston A body kit from 74-76 vintage he also stated he wasn't sure it would act as I wanted it to (a really candid and good guy - I trust his opinion). The folks at SSBC were not helpful at all and I would never purchase anything from them again. We've got 3 examples, my 2 cars and Mike69Cuda car, all of the same era with same characteristics. My system choices are limited by the small bolt pattern and 14 inch wheel which I'm not willing to change. This limits my rotor and caliper size options. You indicate your running the same system are your sure? Normally if you are working with one car and you have a problem its easy to go off in different directions and just start throwing parts and money at it but when you have 3 examples all acting in a similar fashion - - - ?
Well if you're sure everything is correct and you want to tell me how to set up an adjustable proportioning valve, its pretty hard to know what to suggest beyond what I posted above. Go through each part of the system and test and check.
Between the FSM and the Master Tech Conference there is plenty of info.
Yes I am sure I know what a factory fixed caliper system is.
and how they changed over the years.
Between the FSM and the Master Tech Conference there is plenty of info.
Yes I am sure I know what a factory fixed caliper system is.
and how they changed over the years.
To be clear, I did not tell you or anyone else how to do anything. I advised what I did so others would know the steps I took, glad to hear you think I adjusted the manual proportioning valve correctly. The reason I posted this is because I'm at the end of my knowledge and ability and was looking for help, so to tell me to go through it again is not real helpful. However, your example has me wondering about the booster and if there may be a limitation with it. I don't see anyway to adjustment other than the pushrod between the booster and MC, which I did check and was correct according to the tool i referenced. Anyone have any idea why the booster would limit braking or how I might adjust?
Inertia
Well-Known Member
If you brought it to me, the first thing I'd do is take it for a test drive and verify the pedal has limited travel.
I may check for air, but you said bled .
The next thing I'd do is set the emergency to drag while driving.
This moves the shoes into contact with drums to remove any travel to get the shoes to apply. Now ANY hydraulic movement by the m/c should have an immediate result somewhere, fr/back, if not, find out why.
Next drive it, and I would expect the pedal to be "higher", right there, and braking to be improved.
If no difference, as mentioned, open bleeders, press pedal, it should go to floor or vastly farther than now.
If it won't go to floor, find out why, cuz that's why your pedal hits a "board".
If that doesn't give an improvement -
The other thing I would try, is plug the m/c exit ports with no air in the master, expecting that the pedal should be rock hard at the very top, with no travel, if there is travel, find out where.
The booster travel may not be sufficient.
To plug ports, I have nipped off the head of a drywall/shingle nail to be like thumbtack, trim head to fit m/c outlet port, quickly undo outlet tubes, slip the tack into tube and re-tighten tube into m/c, effectively plugging outlets.
Mann , as always, good luck .
I may check for air, but you said bled .
The next thing I'd do is set the emergency to drag while driving.
This moves the shoes into contact with drums to remove any travel to get the shoes to apply. Now ANY hydraulic movement by the m/c should have an immediate result somewhere, fr/back, if not, find out why.
Next drive it, and I would expect the pedal to be "higher", right there, and braking to be improved.
If no difference, as mentioned, open bleeders, press pedal, it should go to floor or vastly farther than now.
If it won't go to floor, find out why, cuz that's why your pedal hits a "board".
If that doesn't give an improvement -
The other thing I would try, is plug the m/c exit ports with no air in the master, expecting that the pedal should be rock hard at the very top, with no travel, if there is travel, find out where.
The booster travel may not be sufficient.
To plug ports, I have nipped off the head of a drywall/shingle nail to be like thumbtack, trim head to fit m/c outlet port, quickly undo outlet tubes, slip the tack into tube and re-tighten tube into m/c, effectively plugging outlets.
Mann , as always, good luck .
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Just a data point. I have had a 67 barracuda 360 4-speed with manual stock KH discs and stock 8 3/4 drums. Stopping was good from all speeds.
Question, because I don't know the answer: If you disconnect and plug the vacuum line, can the brakes be worse than manual? If not, you could try that and see if the high speed stopping problem is still there. Based on my good experience with manual disc front brakes, your leg force should be more than enough.
Question, because I don't know the answer: If you disconnect and plug the vacuum line, can the brakes be worse than manual? If not, you could try that and see if the high speed stopping problem is still there. Based on my good experience with manual disc front brakes, your leg force should be more than enough.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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NOT normal
IDK yet what the problem is but am mulling it over.
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AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
- Joined
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NO NO NO NO.
I have the KH calipers, and have tried every Mopar M/C from 7/8 to 1, and every one of them has worked perfectly. I liked the 15/16 best and so that was where I settled.
I'll give you the short answer first;
This same thing can happen anytime the PowerPiston hits the end of the chamber.
Such as,
1) like you suspect; the M/C runs out of stroke-capacity or
2) the C-ports are obstructed, or
3) you got too much piston knock-back, or
4) the hoses have weak walls, or
5) the discs have too much runout, etcetera,
the point being that for some unknown reason, the caliper-pistons have been forced too far back into their bores. With FOUR of them, this is easily possible.
You can prove this.
During braking just pump the pedal in rapid succession before slamming the pedal down hard. The pedal must return to the top after each stroke to open the C-port, but you gotta get that done before the fluid has a chance to return to the reservoir. If you no longer "hit the board", Badaboom.
that was the short answer.
Now a short explanation;
If you try this with the vehicle running but stopped, you can eliminate knock-back from rotor runout and from loose wheel-bearings.
Once you have ascertained that this is the problem, now you gotta figure out how to cure it.
And now the long rigmarole that Rumble hates;
In the Meantime, I have to ask; is the M/C is plumbed correctly from the M/C to the Combination valve and out to the Calipers. You know that among all the permutations that this can be done in, there is only one right way, .... right?
Ok to be sure;
No matter what type of Valve you have; be it a
Combination valve, or a Proportioning Valve, or a Metering Valve, or even a Y-block, or even if you have plumbed in a manual rear proportioning valve in conjunction with some other valve;
the rearmost chamber of the M/C always goes to the front brakes, with no proportioning of any kind.
Now;
inside the M/C, there is an assembly of two power-pistons mounted on a shaft, one piston for each end of the car. The rearmost is fixed to the shaft, while the front one is free to slide back and forth, located only by the spring between them, when the thing is empty. After filling and bleeding it, That entire chamber is filled with brake fluid, and the frontmost piston will be hydraulically coupled to the rearmost, and they will move together as a unit. Well they're supposed to ...
When you step on the pedal, the whole thing moves forward as a unit, pushing fluid towards the slaves, which are the calipers in the front and the w/c's in the rear. If for some reason, one end of the car has failed, then the whole thing collapses, and is driven mechanically by a stump on one or both of the Power pistons.
Lets say the rear has failed. which is connected to the frontmost chamber. Then both pistons will slide forward, until the frontmost piston hits the limit of it's travel, and then fluid can begin to be pushed out to the calipers. You would feel this in the cab, as additional pedal travel.
Lets say the front has failed. which is connected to the rearmost chamber. Then the hydraulic inter-piston chamber will still drive the frontmost piston in a normal fashion, you just have lousy stopping power, and additional pedal travel. If you had a working Combination Valve, it would turn on the dashlight and inform you of the failure.
Now lets suppose that the hydraulics are working as intended.
You say that you cannot lock the front wheels.
Now you have to figure out if this is a hydraulic problem, or a mechanical problem.
Since you have a high and hard pedal at the beginning of the stroke, AND you don't mention an extended pedal travel, I'm gonna assume the hydraulics are fine; which leaves just a mechanical issue.
You can prove this by removing the calipers and all of the rear brake bits; then installing C-clamps to retain the Pistons and more C-clamps to secure the rear pistons, THEN, stepping on the pedal. If you did not have a booster, the pedal should not budge. But with a booster, there will be some travel. then it locks up high and hard.
But I think we can dispense with that.
That leaves a mechanical issue. In your case I can't think of one unless the pads hang up, which I've never seen these 4-pistons do, or unless the hoses have an intermittent obstruction in them, which I have seen, but never both at the same time..
So let's go back to the master.
In the bottom of the reservoirs are several holes. the bigger ones supply fluid to the power pistons. The smaller ones are for compensating for additional fluid as the pads and shoes wear, and to fill the inter-chamber. In order for these to work, the pistons have to return to their parking spot at the beginning of the bore. and in order for that to happen, the booster has to pull the pushrod back, which then parks your brake pedal up where it belongs. You hear the little thunk when that happens, after every pedal up-stroke, when you take your foot off the pedal. You can tell when the C-ports have quit working because the pedal sinks ever lower over time, and then one day, there is just not enough pedal travel to stop the car. with, eventually, a "hitting the board",
But what if, two things go wrong
1) C-port is not open, and
2) the booster is not properly connected to the pushrod in the M/C.
In this case, the pedal parks just as it always has, but the Power-pistons are running out of travel. So one day you step on the pedal, and the PowerPistons go out, but slam into the end of the M/C before having energized the slaves, or in your case only the fronts may be involved. In this case you have no pedal resistance at the top of the pedal travel, but everything seems normal after that, until you need that last bit of travel in the front system, on a hard stop. When the PowerPiston assembly hits the limit of travel, you feel it like it's "hitting a board," as the hydraulics lock up.
Is that the only scenario?
No. This same thing can happen anytime the PowerPiston hits the end of the chamber.
Such as,
1) like you suspect; the M/C runs out of stroke-capacity or
2) the C-ports are obstructed, or
3) you got too much piston knock back, or
4) the hoses have weak walls, or
5) the discs have too much runout, etcetera,
the point being that for some unknown reason, the caliper-pistons have been forced too far back into their bores. With FOUR of them per side, this is easily possible.
You can prove this.
Just pump the pedal in rapid succession before slamming the pedal down hard. The pedal must return to the top after each stroke to open the C-port, but you gotta get that done before the fluid has a chance to return to the reservoir. If you no longer "hit the board", Badaboom.
If you try this with the vehicle running but stopped, you can eliminate knock-back from rotor runout and from loose wheel-bearings.
Once you have ascertained that this is the problem, now you gotta figure out how to cure it.
For instance, I have the same 4-piston KH set-up on my Barracuda; I took it off a 70 Duster, all of it except the Master, and my booster ended up being from an F-body. I like a lil knock-back cuz my car takes a like a mile to coast down from 65 mph, in neutral, meaning that my brakes are not dragging my fuel-mileage down.
There is a monkey-wrench in this system. It assumes the brake fluid is always a liquid. If at any time it boils, or if it is contaminated with water and the water boils, now you have a gas in the system, which is compressible, and down goes the pedal, with insufficient ability to stop, and bam, you "hit the board". The same thing can happen if the inter-chamber between the PowerPistons, has air in it, but with one difference; in the event that air is in there, you will get a spongy pedal, which you didn't mention.
Ok that's all I got Best of luck to you.
Ok I thought of another thing.
What if the Frontmost PowerPiston was installed backwards, I don't think this is possible, but if it was, then the stump on it would be facing the wrong way and would become a travel-limiter.
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Johnny Dart
Well-Known Member
I'm kind of going through the same issue. Took the 70 Dart out today, and wasn't happy with the stopping power at high speed. Small bolt, manual Kelsey Hayes brakes, 14" wheels. My plan of attack will be to scuff/clean the front rotors, and find the latest, greatest front pads. Then take off the rear wheels, and scuff the shoes and drums, and reinstall/adjust.
Somebody mentioned to you about glazed pads/rotors, and you looked right past it. Could be your problem. News pads, and a normal brake service.
Somebody mentioned to you about glazed pads/rotors, and you looked right past it. Could be your problem. News pads, and a normal brake service.
OK based on discussion and your success without power brakes, I've, disconnected the brake pushrod - Pedal to Booster and travel of pedal is much greater than when connected - so not pedal. So given everything else the limit has to be between power booster and MC. Life is short and I need to resolve this - I ordered a new booster set up from Summit so I'm hoping that eliminates any issue between input to booster to output of MC. Will let folks know result of this experiment. Unfortunately I live in Minnesota and will have to wait until spring to really test but I'm hoping to get a different pedal feel. Will advise!
Sorry, no the brake pads are not glazed - apologize I didn't;'t mean to blow past it
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