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post #1 of (permalink) Old 01-03-2000, 11:07 AM
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Bronco Brakes

I have an '83 Bronco, and lately I have been hearing a hissing noise coming from the pedal cluster area. When I would press down on the brake pedal, it would stop. It kept getting progressively worse over a couple months and then I noticed the pedal would creep to the floor under hard prolonged braking, especially on long hills. That kept getting worse and worse, and now, there is no pressure on the brake pedal. Pumping the brakes does nothing. I checked the brake fluid, the first chamber closest to the front has some bubbles in it, but is at the correct level, the rear chamber has no bubbles and is at the correct level.

Anyone have any ideas what is happening?


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post #2 of (permalink) Old 01-03-2000, 04:11 PM
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Re: Bronco Brakes

It sounds to me like you have a vacuum leak. Your "power" brakes are powered by engine vacuum. First check the vacuum line that leads to the vacuum booster (between the fire wall and the master cylinder). Is the line cracked and leaking? If so, replace it with a new length of vacuum line. Otherwise, you might have a problem with the vacuum booster itself. See if you can detect any leaks. I've never had to replace one of these, but you never know, your's could be bad. A bad vacuum booster or vacuum line line would deplete your degree of brake assist. Is this your truck's symtom? Or are your brakes just not working at all? If your master cylinder has a fluid leak, then you need a new master cylinder. This is a much more common problem. However, it sounds to me like you have a vacuum problem. Having air in the brake lines (a third possibility) can be easily diagnosed. First, start the engine and depress the brake pedal as far as you can with the engine running, make note of how far the pedal drops. Then quickly pump the pedal a few times (again with the engine running). Does the pedal get "harder" (not drop as close to the floor)? If so, then you have air in the brake lines, meaning that you will have to have them bled. A leaking master cylinder will cause you to ingest air in the brake lines. Good luck.

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post #3 of (permalink) Old 01-03-2000, 05:36 PM
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Re: Bronco Brakes

There is a plunger that goes from the brake pedal directly to the back of the Booster(round drum that sits right behind the master cylinder). It sounds like the air leak is coming from there.

I was hoping that a vacuum line fell off or had a hole in it, which would be easy to fix..
It wouldn't surprise me to have both a major vacuum leak and a leaking master cylinder. I think you're right, it probably is the master cylinder.

When I pump it quickly sometimes it gives me some pressure, but it fades instantly, sometimes it doesn't even give any pressure.

Thanks for your ideas!

I'm taking it in tomorrow to have it looked at.

scary drive...

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post #4 of (permalink) Old 01-03-2000, 05:54 PM
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Re: Bronco Brakes

Checking the booster is easy. Run engine, shut it off in a quiet place, listen under dash for a hiss. Assuming good vacuum from engine to booster when engine is running, when you shut it off, the booster holds about 2 &1/2 brake-applications worth of vacuum. So if the booster hisses for a bit, then trails off and stops, you've got a break in the boosters diaphram.
Running the engine, then shutting it off, you should be able to press down on the brake and get a softer pedal about 2 & 1/2 times before it goes hard (to no-power assist).
A final for-sure check if you have or can borrow the equipment, is to disconnect the vacuum line from the Check Valve at the booster under the hood. connect a hand-type vacuum pump (and vac gauge if you have one) to the Check Valve. Put some vacuum on the booster. It should hold it just about forever if the diaphram is good. I troubleshot and replaced one on a full-sized 90 Merc about a year ago. The booster is easy to change, and if that is the only problem, you don't have to disconnect any brake lines. However, in the case that I worked on, the booster went at the output end, where the rod comes out to depress the piston in the Master Cylinder. And the Master had been leaking back there a while. I think the brake fluid eventually softened up the rubber, and it failed. So I put a rebuilt Master in. Toughest problem was getting the #@!**% plastic reservoir off the old one without destroying it! Yeah, the two-big-screwdrivers-and-pry method didn't work!
I think the rebuilt booster was something like $60 at AutoZone.
Have fun, and brake early! UncleDon

post #5 of (permalink) Old 01-05-2000, 01:11 PM
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Re: Bronco Brakes

Danbo, I had the same problem with my 86 Bronco, very easy fix. I bought a new Booster and Master as a unit already put together. I did a bench bleed and installed it. Problem fixed and I got a new Master cylinder too. Taking out the old and installing the new is very easy to do. Let me know if you need any guidance. Kris

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