Hard pedal

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71dart318

71dart318

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Ok so I have a 71 dart with front disks rear drums. Brakes were good when I got it, (I mean pedal normal feel and stopped good). Saw pads were low and MC lid leak, soo decided to do all brakes. First rear shoes and turned drums. Drove car all was good. Pulled it in to do fronts. Needed new rotors, ordered some drilled and slotted. Figured I would replace MC, ordered it. The Mc that was on it had one bigger reservoir than the other. (just figured it was from a later year like 74 or 76ish). My new one came it has the same size reservoirs. I says for disk and drum setup. I have manual brakes. Put it on, new pads, and bleed them. Now the pedal is really hard and it barely stops. The bore of both the new and old are 1.032. And yes I bench Bleed the MC. I had to total collapse all caliper pistons and still had a hard time fitting the new pads, but got them in. Not sure why such bad stopping and hard pedal. Ideas???
 
Make sure the calipers are reasonably centered on the brackets. It is possible to have just one side doing all the work. That will make crappy stopping power. Did you remove the residual valve from the rear port that supplies fluid to the discs? Did you adjust the pushrod to make sure the compensating port is working? If the C-port is not allowing the fluid to return, the pedal will get harder with every application and the brakes will not release. If the calipers are also not shimmed correctly, then your situation makes sense to me.

Some pics of the assemble Scarebird set-up might help
 
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Are the pads binding in the caliper bracket?
The hoses aren't twisted are they? How old are the hoses? They might have collapsed or came apart internally. A fragment of a deteriorated hose can act just like a check valve inside a hose that looks OK from outside.
 
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nothing you have changed should be causing your problem.
..check your front hoses.
 
I would check the master cylinder out again, seeing this happened after changing it out. Check the bore size of your master. If you use anything larger than 1" then you will have an extremely hard pedal. The smaller the bore the more pressure that is output to the wheels.
 
Hoses are old... Just ordered new ones. The bore for most are the same as what I had and this new one. 1.032 Tried to take out pipe seat and residual valve on front circuit and pretty much ruined the new MC. ( still never got seat out ). So now I need another MC. That might be a good thing since I'm doing a 8.8 swap with rear disks. Should I maybe get a 2 to 4 bolt adapter and new style MC? Which one? I know 1 inch or less on bore.
 
the rear reservoir is for the front discs..there will be no residual valve in it.
 
ya I know.. tried to get seat/valve out and tore it up. didn't have a small ez out. Since I'm doing the 8.8 swap with disk in rear what MC would be good? I know I need an adjustable proportioning valve. I think between the old hoses an MC is the hard pedal issue. ( I hope). My caliper pistons were really hard to collapse. Got new seals for them but have put them in since their not leaking. Since I collapse the they wouldn't b stuck or frozen right.
 
Which front discs do you have? The MC with the same sized reservoirs is wrong for both the Mopar 73-76 fronts discs, and for the Kelsey Hayes ones. It should work to make the brakes stop, however.

If your calipers were hard to push in, they very well may have rust in the bores. This is fairly common on the single pistons types. And yes, sometimes if the piston bores are rusty, the pistons will get stuck in the bottoms of the bores, and not move. I'd be pulling them apart. It might be pretty hard if you don't have the MC hooked up. Single piston rebuilt calipers are not all that expensive. If you do decide to pull yours apart, and the pistons are stuck, you will need a special piston wrench. If you rebuilt your self, get ALL the rust out. If the bores are heavily pitted then the chance of success goes down.
 
BTW, I just read that you're going to rear discs. Someone posted on problems a couple of week ago with an issue with them; the standard MC's did not have enough fluid stroke capacity to activate 4 wheel discs. There was another situation like that a week before.

Wilwood/Strange 4 Piston Calipers soft feeling pedal

And not to pile on, but keep an eye on your pads with those slotted/drilled rotors; they can eat pads very rapidly.
 
The calipers are the solid mount not sliding/floating ones. They are 4 piston. I think every issue that I have had you have helped me with nm9stheham. :D:DThank you. Also all you guys are great. Going to tear them apart just to count them out. Then new hoses, finally will tackle the MC situation. For mor fluid capacity couldn't I use an MC from say late 70's dodge truck. They are 4 bolt and bigger resivoirs? Or switch to a new 2 bolt MC from something with all disks?
 
it is very hard to move any caliper piston.By design the piston in the caliper will not slide through the o ring easily.The o ring grabs the piston,distorts or rolls allowing the piston to move about .005 to apply pressure to the pads.When the pedal is released the o ring returns to shape pulling the piston back.This takes very little brake fluid to push the pistons .005
..the pistons do not normally make contact with the caliper bores.
 
71dart318 said:
For mor fluid capacity couldn't I use an MC from say late 70's dodge truck. They are 4 bolt and bigger resivoirs? Or switch to a new 2 bolt MC from something with all disks?
Click to expand...
It's not the capacity of the reservoirs. The big deal in this case seems to be the amount of fluid that the MC pushes to the calipers on a regular braking stroke, with the pedal ratio of the A Body. In the above linked thread, it took a 1-1/8" diameter bore MC to do the job. It'll vary with the specific calipers; so I can't tell you the exact bore needed for your combination.

BTW, when your caliper pistons were hard to push in, were they physically hard or just tricky due all the pistons? If the latter, then maybe they are not such an issue. But if they were physically hard, then I'd be suspicious. In all the calipers rebuilds I have done, I can push the pistons in all the way by hand with no problems.
 
I have done lots of brake jobs and it never took this much effort. Also I had them of the car when I did this. (painted them). So the fluid had no resistance. had to do one piston at a time. Couldn't use my caliper tool or my (go to) C clamp. Had to use a trailer shackle and bolts and then still effort with the ratchet. The old rotors had wear groves only on the backside of each. Going to look up the special piston tool, an get one. New calipers are 130 bucks each soo.. not end of the world.
 
To the OP, if you have the calipers with the black "plastic" pistons, they commonly seize and need replacement, with metal pistoned calipers..

I don't know what you did to ruin the M/C, but the usual manner of removing the seat was to screw an appropriate self tapping screw, (supplied in the M/C overhaul kit, along with 2 new residual valves and new seats), then use a suitable plier, I like the sidecutter, to grasp/pry the screw/seat outta the bore..

I've successfully used 15/16 bore M/C's on everything including 4 piston fr. calipers. I believe if AJ does the math, you'll find that a poorly adjusted 4 wheel, wheel cylinder brake system would use more VOLUME of fluid than it takes to apply all the caliper pistons less than a coupla thou.

Whata ya think AJ, I know you like numbers.. lol

cheers, oh, say 7/8 wheel cyls moving 1/4 inch total..?
 
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BTW, OP, if the MC you had was for drum/drum (which it sounds like) and had a residual valve on the front system, that that might be part of the hard piston retraction problem on the front calipers...... Too bad the MC is kaput.... you could try it again with the pressure off or a bleeder open.

One thing that is being forgot on the caliper fluid volume needed is that there is more retraction at times than just the few thousandths that the pistons seals pull back. Any runout in the rotors or bearing looseness will push the pads back a lot more, as well as any flexing in that are when cornering. (If I have dragging pads, I just makes a few hard turns and voila...they are free.)

This side play does not exist in drums (but there is drum runout) so just thinking about .005" seal retraction is not close to realistic. And, performance calipers are designed with a lot more seal retraction to avoid pad drag in racing. But this may not be the case for the OP's expected new rear calipers which are stock OEM types; it could have been the case for the Wilwood calipers in the thread I linked.
 
I heard my name; math coming. But really it's a non-issue.
Since I don't know your system I'll go with the Mopar KH 4piston set-ups
Part unos;
Lets say; seal retraction is .0025, and bearing run out allows .0015 knockback on each side, and pedal ratio is 7:1(a guess).
Bore size on these is 1.636. The area calculates to 2.102/ cavity. So;
[ 2.102 area x (.0025stroke plus.0015knockback)] x 4pistons =.0336 in cubic
That x 16.387 =.55 cc
To that you add the otherside, also ..55cc, totals 1.102cc or.067 cubic inch
Now lets go to the M/C. It is listed as 1.03125. This has an area of .8352 sq.inch
For it to displace.067 in cubic,it would have to travel .067 /.8352 =.081 inch travel. That is just about 5/64 inch of piston travel,inside the M/C. that's a fat sixteenth.
Add the C-port allowance,(during which no braking takes place), maybe .125, that totals;.206 inch
With a 7:1 pedal ratio that is 1.44 inch pedal travel. Add a bit for hose expansion,system flex, and pedal bushing slop; and, I get less than 2 inchs pedal travel, for sure.
This KH system works very well with a 15/16 M/C.(I know cuz I'm very happy with mine)
15/16 is .6903 area. So then .067/.69 =.097 inch travel or about 20% more. .097 is a tad over 6/64. Go get your tape measure to get an idea of what that looks like.Again adding .125 for the C-port I get .222 inch
With the same 7:1 pedal ratio this is now 1.55 inch pedal travel.Plus allowance for expansion and flex; and still similar pedal travel,say a tad over 2 inches.
-Now, to get a sense of this; the last time I bench-bled a m/c, I'm gonna guess the piston stroke was over a half inch: I'll guess .600.If you subtract, IDK, say 1/8 to get past the C-port. You are still left with .475 inch. So the math on this says the M/C has nearly 5 times as much stroke as it requires. That would be nearly 400% additional travel.
What this shows is that even with generous seal-retraction, and sloppy loose bearings(total .003), almost any Mopar M/C can displace enough fluid. And the reservoir size has nothing to do with the equations; only time to top up.
To this you can add some rotor run-out;although I can't imagine why you would put up with that.
But yeah rotor run-out can wreak havoc on fixed caliper systems. It doesn't take much run-out to kick the pistons back, and then it takes extra pedal travel to affect braking. If you are aware of this extra pedal travel (never mind the hammering going on),and you don't do something about it: don't be blaming the system.

Part deux;
So lets talk about drum brakes;
Lets say the front W/Cs are 1-1/16, area is .8866 each. Lets guesstimate that the total piston travel on one side, is .150(yes I'm guessing) so the volume is .8866 x .15 =.133 cubic inch; add the otherside and you get .266 inch/cubic;or 4.36cc. This is 4.36/1.102 =about 3 times as much as the disc requirement.
Do you still wanna argue that a drum-drum M/C won't work on a disc/drum car?
I wish this idea/argument would die.

Do the math, find exact figures, prove me wrong. You won't hurt my feelings.I will say sorry, if I erred. There's a few guesses in here.
 
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In some ancient mopar magazine, I read that one of the reasons for switching to floating caliper brakes was due to corrosion problems (mostly in the rust belt) and sticking pistons in the KH calipers.
Each piston must move equally to the others, and there was less total movement of the pistons to keep them freed up.
My dad's 76 Volare with single piston discs had one lock up when it was only 5 years old. We did get it apart, and the aluminum piston was not very pretty, but worked fine after cleaning the corrision on it and the bore. It's hard for people to understand just how much road salt was used back in those days. They plowed less, spread a lot more salt. Everyone got where they were going, but your car fell apart in 8 years if you parked it in a garage during the winter.
 
AJ/FormS said:
I heard my name; math coming. But really it's a non-issue.
4.36cc. This is 4.36/1.102 =about 3 times as much as the disc requirement.
Do you still wanna argue that a drum-drum M/C won't work on a disc/drum car?
I wish this idea/argument would die.

Do the math, find exact figures, prove me wrong. You won't hurt my feelings.I will say sorry, if I erred. There's a few guesses in here.
Click to expand...

Thanx Very Much AJ..

May I refer to it everytime the subject comes up?

It's comforting to have numbers back up common sense, and what years of experience teach you.

This example was not to include fancy "race retracting pads/calipers", just what we normally use with nominal runout.

cheers
 
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Darthomas
Unless the bellows is perforated, corrosion inside the caliper is from inside the caliper.
IMO;It is very nearly impossible for moisture to enter the caliper's pressure chamber, from an external source.

IMO The KH design never had a problem. The problem was the equipment of-the-day,required to machine the rotors in the field. It had a really hard time maintaining parallelism.
And yes, the pistons did stick from time to time. But that was moisture related. So how did the moisture get into the caliper? I will always maintain that somebody; let it get in there, or put it in there.
Why would I say that?
Two reasons. 1) brake fluids of the day, and their containers, and the careless attitudes of persons servicing those systems, all conspired to allow moisture to enter from the M/C, and 2) lack of knowledge.
On what basis?
One reason; I put Silicon fluid into my virgin system in 1999. It's still in there.The M/C has a sealed bellows. My KHers still work perfectly.
Oh, and I machined the rotors myself, on ancient machinery.

I like the fixed caliper system. There are no caliper ways to require periodic repairs, and there are no slider pins to seize,or bend, and pads are a drop in deal.
Fixed calipers are simple. No moving parts.They spread the clamp load over a very generous portion of the rotor. The large pad size lasts a long time.
What's not to like? IMO, on a streeter, this is a winning recipe.

Please note the extensive useage of IMO. I would have typed IME;E for Experience, but I have only worked on brakes for maybe 20 years.And I didn't work on brakes every day.I was a general mechanic, and not a specialist. Some fellows here have much more experience. So the punch-line is this; I have had very good success with KHers; your results may vary.
 
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AJ, overall good work. I came up with 2cc's front and 3 cc's rear with a given set of what I think are good assumptions for the fluid into discs and cylinders; I used more seal retraction with a single piston system, 1" wheel cylinders and a tad less wheel cylinder travel. So the same ball park. But, change the assumptions a modest amount, and your or my results for the capacity needed can reverse, with discs needing more than drums. All the numbers we have used are grabbed from the air and not measured. It is too much of a stretch to make any hard conclusions for all situations from such numbers.

If I have it right, the MC piston for the front brakes has to push the distance that pushes the fluid needed for both front and rears. If I read your work right, you used only the front caliper volume to work out the MC stroke. Please check me out, but I think that yields a 4:1 error in the fluid movement portion of your MC stroke computation if one uses your 3x larger wheel cylinder fluid volumes. And it may have to push even more stroke to get the comp port on the rear chamber covered too if it closes later than the front port to possibly add more MC stroke. (I have no clue about the sequence of comp port closures however.)

And from direct experience with going from rubber lines to stainless braided lines, I can honestly say that the amount of fluid needed due to line expansion can be quite large. Just from the sensed pedal movement changes in those cases, it can be as large than the caliper and wheel cylinder fluid capacity for moderate to hard braking (like racing).

BTW, I don't think anyone one said that a drum/drum MC wouldn't work a disc/drum setup. Re-check the postings above. (And I did not go back and change a word.) I did say that there was a case for disc/disc where a larger MC bore fixed the problems and provided the link to that. The point was to try to give the OP a head's up on this potential issue as he looks at rear discs.

The older , short height, small reservoir MC's for drum/drum don't have the capacity to keep filling the space behind the caliper pistons as the pads wear, but as you say, that only has to do with long term wear, not for an individual braking stroke.

And let me bold a few words too.... a few words
 
nm9stheham said:
AJ, overall good work. I came up with 2cc's front and 3 cc's rear with a given set of what I think are good assumptions for the fluid into discs and cylinders; I used more seal retraction with a single piston system, 1" wheel cylinders and a tad less wheel cylinder travel. So the same ball park. But, change the assumptions a modest amount, and your or my results for the capacity needed can reverse, with discs needing more than drums. All the numbers we have used are grabbed from the air and not measured. It is too much of a stretch to make any hard conclusions for all situations from such numbers.

If I have it right, the MC piston for the front brakes has to push the distance that pushes the fluid needed for both front and rears. If I read your work right, you used only the front caliper volume to work out the MC stroke. Please check me out, but I think that yields a 4:1 error in the fluid movement portion of your MC stroke computation if one uses your 3x larger wheel cylinder fluid volumes. And it may have to push even more stroke to get the comp port on the rear chamber covered too if it closes later than the front port to possibly add more MC stroke. (I have no clue about the sequence of comp port closures however.)
There is only one C-port. The rear chamber gets its fluid by bypassing the one-way valve in the center of the piston stack.

And from direct experience with going from rubber lines to stainless braided lines, I can honestly say that the amount of fluid needed due to line expansion can be quite large. Just from the sensed pedal movement changes in those cases, it can be as large than the caliper and wheel cylinder fluid capacity for moderate to hard braking (like racing).
Once the C-port is passed, no more fluid can enter the line. If hose flex is excessive, it shows up as a longer piston stroke and subsequent pedal-travel, and a springy feel. Since we are only using 1/4 of the piston travel to set the pads onto the rotor, we still have3/4 of the stroke available for system flex.Thats why there is six or more inches of total pedal travel available. Well that and for compensation to poorly adjusted drums.
Teflon lines reduce/eliminate a lot of system flex, and give my ride a nice confidence-inspiring, hard-pedal. .


BTW, I don't think anyone one said that a drum/drum MC wouldn't work a disc/drum setup. Re-check the postings above. (And I did not go back and change a word.) I did say that there was a case for disc/disc where a larger MC bore fixed the problems and provided the link to that. The point was to try to give the OP a head's up on this potential issue as he looks at rear discs.
This comment is not related to this thread, but rather to all the posts of bygone-days.

The older , short height, small reservoir MC's for drum/drum don't have the capacity to keep filling the space behind the caliper pistons as the pads wear, but as you say, that only has to do with long term wear, not for an individual braking stroke.
that is correct.
I would not use just any old M/C for road-racing, or autoxing. But for a summer-only streeter? How many miles does a streeter put on in a summer, and what percentage of those are pad-wearing miles? I have a M/T with plenty of compression braking. I drive defensively,cuz I wanna drive my car all summer;not have it in the bodyshop for weeks to months. I pick my times and spots where Ima gonna get stupid. I have had O/D, almost since day-1. What Ima getting at, is I use my brakes very little. Subsequently, my pads last a looong time. Some might say I drive like a grampa. My brake system has accumulated 125,000 miles on it since year 1999. I won't tell you that I'm still on the first set of pads, or that I have never had to add fluid. That would be preposterous. lol
And let me bold a few words too.... a few words
Click to expand...

lol,Thanks for chiming in.
Click to expand^^
I did the math for the rear cylinders, but the post seemed to be getting a little long, (lol) so I edited it out. Since the rear brakes have their own chamber, and the shoes wear so slowly,I deemed it File13.
 
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inertia said:
Thanx Very Much AJ..

May I refer to it everytime the subject comes up?

It's comforting to have numbers back up common sense, and what years of experience teach you.

This example was not to include fancy "race retracting pads/calipers", just what we normally use with nominal runout.

cheers
Click to expand...
Not sure what the ramifications might be to my saying yes, but I'm here to help, so if you can make use of anything I say, quote away; just make sure I didn't err in what I said. Nobody want's to quote an error. Well except for theoretical scientists; they like to perpetuate myths and untruths presenting them as facts,lol. My heart aches for our kids
 
Well so far got driver side caliper tore apart. It looks pretty good(as far as no corrosion or stuck pistons). But was filled with a lot of black sludge. Also I learned when you paint your calipers don't paint where the pads sit. (dumb move) Had to tap the pads out of caliper on the bench. Going to put in the new seals and boots remove paint on the pad contact surfaces, then reinstall. Then work on passenger side.
inertia said:
I don't know what you did to ruin the M/C, but the usual manner of removing the seat was to screw an appropriate self tapping screw, (supplied in the M/C overhaul kit, along with 2 new residual valves and new seats), then use a suitable plier, I like the sidecutter, to grasp/pry the screw/seat outta the bore..
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I didn't know how to get them out. Used some odd methods an made a mess of it. But I agree with nm9stheham that it wasn't the right Mc anyway. Now that I'm going to need a new one, trying to figure out the right one for 4 wheel disks.
 
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