'Grabby' brakes

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nm9stheham said:
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.

May have to bite the bullet and turn the drums and get shoes, cheap insurance..

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.

I put new Wagner brand OE pads on the front.

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.)

So simple, it's genius.. I'll test this way as well to try to isolate the issue.
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clifftt said:
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.
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Not sure I can answer this one. It was spec'd as a direct replacement, but I wouldn't put any money on a mass-reman'd part being application correct.. Can this be checked by -part#? Or by measuring the 'pushrod end' without disassebling the MC?
 
nm9stheham said:
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.
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I really do like my brakes to work well, all the cool, fast stuff can wait until it stops 'perfect', then handles well.. I just don't want to sound whiney because I want the brakes to be near perfect..
 
And thanks to all you guys who have chimed in. I appreciate the input from folks that know their stuff, it's obvious you guys do. I'll check out each avenue you guys have pointed to and let you know how each goes. Thanks again and I love this forum...
 
Ben Drinkin said:
The 69 had manual 9" drums. The 73 had power front discs with rear drums on a 7.25 rear end (I didn't check what size the rears were)..
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Did you change the pushrod from the pedal to the MC.

Power brake and manual brake are different lengths.

If not, it may be your problem.
 
Bad Sport said:
Did you change the pushrod from the pedal to the MC.

Power brake and manual brake are different lengths.

If not, it may be your problem.
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Thanks for the thought. I changed everything, pedal, brackets, pushrod.. I even used the hard lines from the donor.. And the one to the rear brakes was a *****.
 
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.
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She performed this test flawlessly. Pedal dropped about an inch and held firm..
 
Well, theres that then.
Ima still leaning towards a too-long pushrod.
But I am also considering a mismatched front/rear system, or as a last-on-the-list-item, a faulty P-valve.
If the rears are still 9 inchers:
Heres how that could play out. Disc brakes are not self energizing. That means they require a lot of pedal pressure(compared to a drum brake). So the factory usually puts a booster on there. Makes it easy for grampa(me). Now the rears are still self energizing. And say they are 9 inchers. It doesnt take a lot of hydraulic pressure to lock those babies up.So when you lay on some pedal pressure, the booster does its job and multiplies your leg pressure. The front may work fine. But if the rear receives too much pressure, it will tend to lock up those little 9s.
Since the rear is destined for replacement, Im going to throw this out there.If you somehow prevented brake pressure from activating the rear brakes,you might learn a valuable bit about the source of your grabbiness.And if its coming from the front, someone has already posted a way to prove its booster(pushrod) related.
 
nm9stheham said:
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.)
Click to expand...

Disconnected the booster completely. Brakes are smooth all the way through (albiet with ample foot pressure!) I'm guessing pushrod adjustment next? I destroyed my vacuum hose in the process tho. Got to get some new hose tomorrow, i have some new 3/8 fuel line but i want to get some good strong vacuum hose.
 
The reaction valve is the brain of the booster. If the pushrod length is right, the brain accepts your leg input and tells the booster how much boost to add to your leg pressure. If the P-rod is too long it will boost too early. If too short; too late. This is a mechanical device, and therefore stupid. It only responds to the push-rod length.Its possible to break it with a very-much-too-long adjustment. If its waaay too short(never tried it) it may not boost at all, but I doubt that.
Inside the booster is a big rubber-type diaphragm, running down the center. On the one side is atmospheric pressure.With the engine running and connected up, the air on the other side gets evacuated. So now we have a pressure differential in there. Which we can use to perform work. In the center of the diaphragm is stuck the reaction valve.When the P-rod acts on it, it re-acts by adding its boost to your leg pressure.
 
Ben, Ben
Did I not say the reaction valve is stupid.
Of course its worth playing with. How do you know a replacement wont also require adjustment?
And you may have to adjust it several times, to get it where you want it. It will be fun. And when its done, you will have the satisfaction of being the only guy in the neighborhood who knows how.
 
AJ/FormS said:
Ben, Ben
Did I not say the reaction valve is stupid.
Of course its worth playing with. How do you know a replacement wont also require adjustment?
And you may have to adjust it several times, to get it where you want it. It will be fun. And when its done, you will have the satisfaction of being the only guy in the neighborhood who knows how.
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I love it. I'll start stretching my rod a little at a time until i get there! Thanks for the help and the laugh.
 
Um, in post 35, I was leaning towards a too-long current setting. So... would that be stretching? Or....
Let the first adjustment be no more than 2 or 3 flats.After that 1 flat.
I suppose to speed things up, you could put shims between the m/c and the booster, right at the mtg bolts. This would act the same as shortening the rod.Do not overtighten.Then when you get it right, just measure,average, and adjust. Shazzam!
 
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