'Grabby' brakes

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Ben Drinkin

Ben Drinkin

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I swapped front and rear suspension from a 73 Swinger into my 69 barracuda. Discs,booster, master cylinder, combination valve..... had the rotors turned (thay didn't really need it), i put new seals in the calipers, new braided lines to the calipers, flushed all the hard lines. I used a rebuilt mc/booster from o'reallys. All i did to the rear brakes was clean and inspect, they looked great. The brakes work just fine, but it feels like there's a real fine line between 'lightly hitting the brakes' and the point where they grab and act like like a 'panic stop'. I've gotten used to driving it and it's not horrible, just not as smooth as I'd like it to be. Is there something i should look at or am i just being a picky ***** wanting it to stop like a 2015 car?
 
Did you reuse the booster from the drum brakes? If so there is your issue. MT
 
Yes that sounds like a booster issue. Actually it sounds like a M/C pushrod adjustment issue. Get that thing adjusted right you will have very nice brakes.Its also possible that the mechanism that controls the boost inside that booster is busted. So try lenghtening that p-rod about a 1/32, and see what happens.There is a spec for that, but I dont have it.
It sounds like there no boost, until you have pedaled it quite hard and then the boost comes all in. By lengthening the pushrod, the boost will start sooner and softer.If you cant find a spec. it may take several tries to get it right. The M/C just needs to be unbolted from the booster,slid forward a few inches, and the adjuster is right there.

edit; on the MidlandRoss single diaphragm unit its .915 +/- .005.
This is from the mounting plate (your plate is on there,right?) to the top of the acorn-nut adjuster. The MR unit is the large diameter unit with the band-clamp holding the two halves of the chamber together.
 
Also a good idea to check the rear wheel cylinder diameters.

I'm convinced Mopar did a lot of the "proportioning" with the bore size in at the wheels.

My 66 Coronet manual rear drum car had enormous rear wheel cylinders compared to my 87 5th Ave power disk donor car.

Smart to keep all the matching components together.
 
Thanks for the replies. To clarify, the rear brakes, front brakes, prop vale, booster and MC are all from the 73 Swinger, no intermixing of 69 and 73 parts. I replaced the booster/MC with a unit from O'Really's spec'd for a 73 Swinger.

Like i said the brakes are pretty decent, just not as smooth as I think they ought to be. I'll try lengthening the pushrod a hair at a time and see how that does..

Thanks again for the input. This forum is the greatest..
 
Or they sold him a drum brake booster. Could be. MT:burnout:
 
Mopar Tim said:
Or they sold him a drum brake booster. Could be. MT:burnout:
Click to expand...

Looks identical to the one that came off the 73. Brakes worked fine on the Swinger, I kept the old booster and MC in case I ran into a 'bum rebuild' issue, which I understand is rather common.
 
Even when new, the Chrysler power brakes were notorious for being touchy.

I like things simple so I'm running 73-76 disc brake with 10"x2-1/2" in the rear MANUALLY on my 68 cuda. Unless you have a physical condition that limits your leg strength, I would recommend the same for you. A 73-76 power brake MC run manually (no booster) has a 7/8" bore and results in excellent linear feel. You would need a manual MC pushrod with this setup or an aftermarket adjustable one. Everybody who does this likes it.
 
KosmicKuda said:
Even when new, the Chrysler power brakes were notorious for being touchy.

I like things simple so I'm running 73-76 disc brake with 10"x2-1/2" in the rear MANUALLY on my 68 cuda. Unless you have a physical condition that limits your leg strength, I would recommend the same for you. A 73-76 power brake MC run manually (no booster) has a 7/8" bore and results in excellent linear feel. You would need a manual MC pushrod with this setup or an aftermarket adjustable one. Everybody who does this likes it.
Click to expand...

Thanks Kosmic. The only physical condition I have is that I'm a giant sissy.. If a simple pushrod adjustment doesn't smooth things out, I may very well do away with the booster.
 
A hard pedal = no air.
Heres a quick booster check..
Jump in the car and stab the brake pedal about 8 times to bleed any vacuum out of it that may be in there. Lay about 50 lbs of pressure on the pedal and start the car. If the booster is good and adjusted right, the pedal will drop down under your foot,about 1 inch,with no help from you, as the vacuum builds up in the chamber.
 
I had this same issue once, stumped me for a long time.
Maybe you have the same? I checked all the usual's as mentioned by others, I sure thought it was the booster....
My trouble was, when cutting the rotors, they were cut beyond the allowable limit just barley. and I had not replaced the rear shoes. So when applying brakes, the master cylinder plunger traveled so far (cut on the rotors) that when the rears came on (with old and possibly worn) shoes/ drums they locked up. If you haven't already, check the specs on the rotors and the drums, install new shoes.... bet your ok after that. ...worth a look at least :)
 
YY1 said:
Also a good idea to check the rear wheel cylinder diameters.
I'm convinced Mopar did a lot of the "proportioning" with the bore size in at the wheels.
Click to expand...
ALL brakes system designs use the wheel cylinder and caliper piston diameters as a fundamental part of the proportioning; it is the main thing used to set proportioning; the prop valve varies it from there.

To the OP: I agree with the comment on the brake shoes; they may look good, but you don't know how hot they have been in the past or it they got soaked in brake fluid or axle fluid; that can effect coefficient of friction of the shoe material and that effects braking all over. Similarly for the rear drums; they need to be turned, IMO.

Did you put new pads in the fronts? The pad material relative to the rear shoe material is another item that sets basic brake system F/R proportioning, and who knows what you got off the older car.

Another way to check the booster effect on all of this is just disconnect the vacuum line to it and plug it where it goes into the intake. Bleed off the residual vacuum in the booster with the multiple pumps of the pedal and then go drive and see if the grabbiness is still there or not. That will separate the booster from the rest of the brake system as the issue. (Be a bit cautious 'til you get the feel for the non-boosted brake action.)
 
Just say'in its worth a check.... cost zero

I went through three boosters with the same issue...I though the same as you guys booster, ....turned out...worn drums and undercut rotors out of spec, and shoes on the rear that were old and getting close. Push the brake and GRAB, instant lock-up. A measure is cheap..easy and why not. I could be wrong...but that's my opinion , never say never, especially if someone's been there.
 
Mark
I can think of at least 3 different scenarios that could have manifested the same symptoms that you experienced; 1)misadjusted rear brakes, 2)incorrect set-up leading to a non-functioning compensating port, and #3) pistons cocking in the bores, driving the leading edges of the pads into the rotors.
As a auto safety inspector for the govt of Mb(ptoui) I have seen many worn-out/under-spec rotors, with nearly worn out pads. And they stopped pretty good. Many owners had no idea. I have personal cars that had under-spec rotors, and worn pads, that stopped just fine.
I have had cars in my bay, where the rear adjusters had not worked for so long that the pistons finally came out far enough to puke fluid all over the place, and the owners had no idea there was a problem, until the pedal dropped.
AS you probably know, rear brakes are self-energizing. I can certainly see rear brakes being grabby if they are doing all the work.Especially if the drums and shoes are so worn and/or out of adjustment that the pushrods end up being dragged around by the self-energizing system.The question then is;why are the rears being asked to do all the work? As long as the C-port is working the M/C will pump fluid out to the front calipers.The M/C doesnt know if the rotors are "barely"undersize. And once the fluid is out there,barring other mechanical problems,That end will work just fine.
What Im suggesting is this; with your symptoms, I draw a different conclusion. Im not speaking to whos right or whos wrong.I wasnt there.I cannot possibly know Im right.
 
Is the replacement master cylinder the same bore diameter as the '73 Swinger you started off with? Smaller diameter, easier to push, easier to lock 'em up.
 
Ben Drinkin said:
Looks identical to the one that came off the 73. Brakes worked fine on the Swinger, I kept the old booster and MC in case I ran into a 'bum rebuild' issue, which I understand is rather common.
Click to expand...

They can both look the same. MT:burnout:
 
One more note to the OP: If the brakes are grabbing like in a panic stop with pedal pressures in the non-panic range of use, then you are not being a 'picky *****' at all. There is something for sure wrong. I drove these A bodies for 10-20 years 'back in the day' as my regular car and there was nothing 'grabby' in the brakes, period.
 
AJ/FormS said:
A hard pedal = no air.
Heres a quick booster check..
Jump in the car and stab the brake pedal about 8 times to bleed any vacuum out of it that may be in there. Lay about 50 lbs of pressure on the pedal and start the car. If the booster is good and adjusted right, the pedal will drop down under your foot,about 1 inch,with no help from you, as the vacuum builds up in the chamber.
Click to expand...

I'll give this a whirl when I get home from work tonight (provided I'm not putting in another 16 hr workday, yesterday kicked my butt).
 
marksmopar1 said:
I had this same issue once, stumped me for a long time.
Maybe you have the same? I checked all the usual's as mentioned by others, I sure thought it was the booster....
My trouble was, when cutting the rotors, they were cut beyond the allowable limit just barley. and I had not replaced the rear shoes. So when applying brakes, the master cylinder plunger traveled so far (cut on the rotors) that when the rears came on (with old and possibly worn) shoes/ drums they locked up. If you haven't already, check the specs on the rotors and the drums, install new shoes.... bet your ok after that. ...worth a look at least :)
Click to expand...

I had the rotors trued up, took off very little. Still lots of meat on the rotors. I put new pads on the front, did nothing but clean up the rears, appeared to be in great shape (but nothing was measured and shoes were not replaced. Planning to replace the 7.25 so I didn't put much thought into the rears.)
 
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