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post #1 of (permalink) Old 04-14-2000, 08:49 AM
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Need help with 86 brakes low pedal

The problem I am having is low brake pedal, I have replaced the master and the brake booster, beed the brakes over and over and the pedal seems great until I start the truck then the pedal will travel more, heres what I have done, let me know what I am missing.

1. replaced master twice to make sure it was not bad
2. replaced the booster twice """
3. bleed many times with the truck not running.
4. all pads have more then half on them
5. not leaks
6 has great pedal when bleeding until i start the truck,

Need your help guys, have been working on my own brakes for 15 years but this one has got me............

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post #2 of (permalink) Old 04-14-2000, 09:34 AM
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Re: Need help with 86 brakes low pedal

Check your vacuum lines. With the rubber being the way it is, you could be losing some boost.

post #3 of (permalink) Old 04-14-2000, 09:56 AM
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Re: Need help with 86 brakes low pedal

A couple of other things are:

1.Did you replace the rear shoes or take them off the backing plates? If you did, it is possible to mix up the the left and right self adjusters. If the adjuster is on the wrong side, it will loosen the rear shoes instead of tightening them. This causes the pistons in the slave cylinders to have to move farther and takes more peddle stroke. Things will pump up fine while bleeding, but since the springs pull the shoes back which pushes the slave cylinder pistons in, you lose all of the extra fluid needed for the extra long piston stroke. All that extra fluid is pushed back into the master cylinder and on one peddle stroke, not enough fluid gets down to the slave cylinder. I found this working on a friends truck.

2.There is usually an adjustable ball on the end of the booster rod that pushes against the master cylinder piston. There should a very small gap between the two.

A couple of things to check for.

If it ain't broke, I can probably still fix it.
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post #4 of (permalink) Old 04-14-2000, 10:02 AM
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Re: Need help with 86 brakes low pedal

I DID CHECK THE ADJ. OF THE REAR BRAKES AND ADJ. THEM PRIOR TO BLEEDING. I ALSO CHECKED THE BOOSTER PUSH ROD FOR
CORRECT LENGTH, ACCORDING TO THE BOOK IF YOU PUSH DOWN ON THE PEDAL SAY 1/2" AND FLUID MOVES IN THE MASTER YOUR RIGHT ON.???????????????

post #5 of (permalink) Old 04-14-2000, 01:58 PM
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Re: Need help with 86 brakes low pedal

Unless there is fluid leaking from the system, there can only be three basic possibilities.

1.The master cylinder is not pushing enough fluid from the piston.
2.The wheel cylinders and calipers require too much fluid for one peddle stroke.
3.There is air in the system that is being compressed.

You have bleeded the brakes and it sounds like a couple of times. I'm sure you can see that there is no air in the lines so we can rule out number 3. Since you have fluid passing through the master cylinder with such little peddle movement, it sounds like you are getting a full stroke on the piston. That means that enough fluid is moving through the lines. From the amount the bleeding that you have done, we've passed the bench bleeding stage. This rules out number 1. Now there is no adjustments for the front calipers. They are either frozen or leak. A frozen caliper will not cause the peddle to go to the floor, but a leaking one will. You would have seen the puddle, so this rules out the front calipers. This leaves the rear brakes. Since you have adjusted the rear brakes and know that were set correctly before the bleeding and driving, check them again. If the shoes are backed clear off, that will be a problem.

If it ain't broke, I can probably still fix it.
post #6 of (permalink) Old 04-14-2000, 03:16 PM
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Re: Need help with 86 brakes low pedal

OK We'll give it one more shot tonight, the pedal goes about 3/4 of the way to the floor only when the car is running. No vacum leaks .
Nothing in leaking ????????

Let you know THANKS GUY'S

post #7 of (permalink) Old 04-14-2000, 04:52 PM
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Do they pump up?

When its running and you pump the brake peddle, if you keep pumping, will they eventually pump up? If they do, I'd imagine that you lose the pressure after a minute or so and the peddle will go to the floor when you step on it. The system probably acts like a bad master cylinder, which is why you replaced it and the booster.

I am guessing on that base on an experience on a friends rig. He put rear shoes and front pads on everything looked fine. He did a routine brake bleeding and line flush. Went to test it and the peddle went to the floor, barely stopping the rig. Found that if he pumped the brakes three or four times, he could retain enough pressure to make the brakes work. Thought he got air in the lines and blead them again. Got a little air, but not enough to cause a problem and still had the same problem. He then rebuilt the front calipers and rear wheel cylinders, which meant that he had to take the rear shoes off again. He noticed the adjusters were screwed all the way down and were frozen. He freed and lubed them thinking they rusted together because he forgot to lube them after cleaning them on the first brake job. Put everything back together, blead the brakes and had the same problem. He then replaced the master cylinder and adjusted the ball on the booster rod, bench blead the master cylinder, installed it and blead the brakes again. I grabbed my power bleeder thinking that he wasn't doing a good job. No air, so got to thinking that for some reason the slave cylinders (front calipers and rear cylinders) were requiring more fluid than the master cylinder can give. Now the front caliper pistons don't slide far enough back into the bores when the peddle is let up. This puts the front brakes out of the equation. But the rear wheel cylinders will because of the return springs on the shoes. So I pull a drum off and found that the adjusters were screwed all the way in again. This meant that the rear shoes had to move quite a ways before they would touch the drums and that extra movement required the pistons to move quite a ways. That extra movement required more fluid than the master cylinder could give and the peddle went to the floor. I swapped the sides the adjusters were on and the brakes have worked fine ever since.

All that work because of a stupid adjuster.

If it ain't broke, I can probably still fix it.
post #8 of (permalink) Old 04-14-2000, 06:20 PM
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Re: Do they pump up?

86BRONCO - FYI, if you have your foot on the brake and pressing down, then start it, the pedal WILL normally drop some. Quite noticeable. Because the running eng. now provides vac., and the booster can now multiply your effort.
Is there some other symptom than that that makes you think something is wrong?
Just an observation.

post #9 of (permalink) Old 04-16-2000, 05:57 PM
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Re: Do they pump up?

Yes......I was curious as well. If I've read everything here it sounds like:
You have a perfectly functional and reliable (read good hydraulic integrity) brake system UNTIL vacuum is applied to the booster and you lose your pedal. This sounds like the ratio of the length of your pedal in relation to the rod is out of whack.....or the rod is too short and therfore multiplies the, otherwise normal, pedal withdrawl upon start up.

GeeAea

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post #10 of (permalink) Old 04-17-2000, 09:08 AM
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Re: Do they pump up?

Well all the brakes work but the pedal is still low, goes down about half way wich I think is to far after driving my Dads F150, you could stop
that truck with your little toe and the brake pedal only goes down about 1".

Thanks for all the info and help.



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