Bit of a rant about single-piston calipers used on mid-sized and big Mopars

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Bill Crowell

Bill Crowell

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The calipers I am talking about are the single-piston sliding-caliper units, where the outer brake pad reacts against the inner pad via the machined "ways" in the mounting adapter.

I thought my old calipers were dragging too much, so I replaced them while also cleaning and lubricating the "ways", but the new pads drag just about as much as the old ones did.

I notice that in this design the piston seal is the only thing that retracts the pad after the brakes are applied, and IMHO the piston seal just doesn't have enough strength to retract the pad, especially after the "ways" lose their lube and get a little dirty. I don't like this. I would like my front wheels to spin more freely!

I wonder if there is a higher-quality caliper, that retracts better, that would bolt into a big Mopar? (If the answer is obvious, please excuse my ignorance.)
 
Bill Crowell said:
The calipers I am talking about are the single-piston sliding-caliper units, where the outer brake pad reacts against the inner pad via the machined "ways" in the mounting adapter.

I thought my old calipers were dragging too much, so I replaced them while also cleaning and lubricating the "ways", but the new pads drag just about as much as the old ones did.

I notice that in this design the piston seal is the only thing that retracts the pad after the brakes are applied, and IMHO the piston seal just doesn't have enough strength to retract the pad, especially after the "ways" lose their lube and get a little dirty. I don't like this. I would like my front wheels to spin more freely!

I wonder if there is a higher-quality caliper, that retracts better, that would bolt into a big Mopar? (If the answer is obvious, please excuse my ignorance.)
Click to expand...

Noticing the same thing over the years I feel the same way about that.
The very slight roll over of the piston seal seems to be all the return you get, but I have always been hesitant to try and do anything about it for two reasons.
1. They were made that way and 2. I don't want my pedal to have to go down that far to get the pads back to where braking is again.

I kind of always figured it might be safer and simpler to just go with an aftermarket system that may have less drag.
 
Never had a problem with those calipers and many of us have run them for years. Your rubber flex brake hoses maybe collapsing inside. This would not let the fluid return to master cylinder and will hold the brake on. Just a thought.
 
Hellrats said:
Never had a problem with those calipers and many of us have run them for years. Your rubber flex brake hoses maybe collapsing inside. This would not let the fluid return to master cylinder and will hold the brake on. Just a thought.
Click to expand...

Once a turn is made, or a bump is hit they back off more anyway.
If you pull in and stop the car with the brakes then jack it up and pull the wheel is when the most drag is felt.
If you really want to see the "normal" amount they drag, pull in and stop the car with the E brake and THEN check the drag of the discs and it's a lot less.
(as long as everything is ok, and no piston sticking, line collapsing or a master plunger adjusted up to tight.)
 
This is all entirely by design. The instant you increase the angle in the piston seal groove that controls the deflection (and therefore retraction) of the seal and piston, you'll be here complaining about excessive pedal travel. Been there, done that.
(Fyi the groove in the caliper for the seal is not rectangular, it is trapezoidal)

Additionally, the runout of the rotor and deflection in the wheel bearings aid in kicking the pads out. If the pads are truly dragging, you'll smell it in no time.

This is precisely why A12 cars and drag racers use drum brakes.

Smaller pistons and rings have better control over this, so a four piston caliber might be of benefit.
 
Don't 95% of new cars today use that same single-piston, sliding caliper design? In contrast, 1964-72 A's had fixed 4-piston calipers (as did Mustangs), as do many high-end sports cars today. It does seem unreliable that the square O-ring is supposed to rotate and act like a spring to retract the piston, but seems to work well. Post 5 is the first I read of a trapezoidal groove, but that helps it make sense. If your calipers are old, the O-ring may be too stiff. On most cars, rebuilt calipers are so cheap that most people go that route. I rebuilt the calipers on my 1982 Aries, because rebuilt ones were probably expensive then. That might make sense on a 1970's Mopar. The only trick was getting the piston started without cocking in the bore.
 
I've never seen a caliper that did NOT act as described. I would think that keeping the wheel bearings in proper adjustment (no play) and the disks true (no wobble) would be all that is needed. Many types of lining grab when cold, or stop a bit better after being warmed, so you don't want them TOO loose.

Too bad you cant do a 70 mph run long enough the get things "normal," then do a stop WITHOUT front brakes and see how they spin "as if" they were going highway speed.
 
That's why i prefer the pin style...they slide more freely and can actually get noisy...but that's a good thing.No resistance.
If the rattling pads bother you just coat the piston side with that red stuff and although they stay loose they will be quiet.
 
I find that with hi-mileage units, the pad anchors get hammered, and then the pads get stuck in them. I also find that corrosion occurs in the o-ring grooves of the slider pin bores. And then the friction here is stronger than the seal-retraction. I also find on high mileage units that corrosion occurs in the grooves where the square section o-rings are, and this pushes the o-rings hard against the pistons, and seal-retraction is reduced or eliminated.
Some guys grease the pistons prior to installation, to help slide the pistons in. But this pretty much eliminates retraction, cuz the seals do not deflect at all;instead, the piston slides right on thru, and never comes back.
If everything is working right, I just run my front bearings a tad loose, and knock-back takes care of it. And before anyone jumps on me about this; go jump in a lake; I've been running slightly loose front wheel bearings, on all my vintage cars, totaling nearly 5 decades, and have never even heard of replacing them. Not like the crap cartridge bearings on modern strut cars, whose bearings are many times larger and go up to maybe 5 years.
It's nice to be able to slip it into neutral and coast for a mile. That's just one of the tricks I used, to get over 30mpgUS from one of my combos.
 
Running wheel bearings a hair loose is one of the benefits of floating calipers.The early Kelsey Hayes fixed calipers didn't have that luxury.
Corrosion on the slides is easily dressed with a file.
 
Residual valves. 3 psi if i recall correctly. There has to be some drag. Or pedal travel would be excessive.
Wheel bearings-a tad loose is better than too tight.i use a very short pair of slip-joints to lightly snug bearings, been doing them like that for 30 years.
If you were to measure the drag-after driving without touching the brakes you would find near zero drag.
I would stay away from the phenolic pistons,as they wear they like to **** in the bore, then you have drag...then smoke.
 
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