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Old 06-20-2004, 08:10 PM
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Question Vacuum leak from brake pedal?
I just noticed yesterday when my truck is running, you can hear a sucking sound coming from the brake pedal. It seems to change when you depress the pedal. When you shut it off it's most noticable, because you hear it leak down for several seconds. If you depress the pedal then it stops, but seems to pick up again after you release the pedal?

Is my booster going? Is there something else I can look into? The truck only has 43K miles! help!
   
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Old 06-20-2004, 08:12 PM
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A bad booster will do that. The diaphragm breaks down over time, drys out, leaks, etc. and sucks air through the vacuum line and into the intake.

Good luck with it, they're a real treat to change.
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Old 06-20-2004, 09:52 PM
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Thats what I was afraid of. Sounds almost like it's leaking where the rod comes through the firewall. Is that possible? Would it hurt to just leave it? Tommorow I'll disconnect the booster and see if it stops. Could the check valve cause that?
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Old 06-20-2004, 10:38 PM
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It won't hurt to leave it the way it is, you'll just have to push the pedal harder. If you decide to disconnect the booster altogether, you'll obviously want to plug the vacuum line with something.
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Old 06-20-2004, 10:52 PM
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I've had the exact same thing going on with my 86, usually when your booster is going bad it hisses when you push the pedal, but we have it exactly backwards and the brakes work fine don't they? It's weird but I figured, "Ah hell, as long as the brakes work." I've had it doing this for well over a year and my brakes work just fine, I've searched for that leak and it's impossiable to find, but it's under there. I wouldn't worry about it, just turn up the radio. Glad I'm not the only one.
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Old 06-21-2004, 12:32 AM
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Dighphram in the booster is going bad...
My 3/4 ton has a dual diaghphram unit...
I got sick of it hissing and making it idle like garbage, so I put a new one on...
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Old 06-21-2004, 12:41 AM
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82f100 has a good point. Not only will it make you nuts but will also drive your fuel management system crazy. They actually arent all that hard so just bite the bullet and get it done. Took me about a half hour with air tools and the proper amount of liquid refreshment

Take care and cheers hope this helped.
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Old 06-21-2004, 12:42 AM
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oh, btw, just diconnecting and plugging the vaccuum line wont do it if the diapgragm is gone. The air is leaking around that.
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Old 06-21-2004, 01:13 PM
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I just replaced my vacuum booster and master cylinder ($131 from NAPA) and it cured the hissing plus the truck starts and runs better now. Must have been sucking air back into the intake.

Now gotta adjust master cylinder push rod as front disks seem to be binding more than normal. Anyone have any suggestions on this problem?
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Old 06-21-2004, 03:03 PM
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I guess it's probably the booster. I pulled the check valve out and put a cap on it. Started it up and I didn't hear the hissing at all, even after I shut it off. Then when I pulled my cap off the check valve I heard it hiss, so the line and valve are probably ok. It looks like I can get just the booster for about $80, or with the master for $125. If I change it should I get the whole thing? I think I could probably leave the master and just change the booster.

Also, my ford shop manual shows a bendix single diaphragm and tandem diaphragm unit. What is the difference, how do I know what I have?
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Old 06-21-2004, 04:24 PM
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I took it out and made a plate to cover the holes. Manual worked until I had the money for a new one. Mine actually rusted through.
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Old 06-21-2004, 07:35 PM
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So it shouldn't be too involved to dig into this? And what about the two different boosters in my manual? napa only lists one.
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Old 06-21-2004, 08:12 PM
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I am pretty sure they both will work. The newer one is smaller in size but it will bolt up.
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Old 06-21-2004, 08:51 PM
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Your manual likley also shows the Dual diaghphram unit found on the F250HD and 350.
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81 F100/86 F250 HD 4x4, 6.9/T19/D44 TTB junk/10.25. All smoke, no power, turbo no more...
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Old 06-21-2004, 10:35 PM
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Yep, just looked at my manual more carefully, the single style is 150-250 and the other one is for 250HD and 350, or something like that. Anyway, only one for my truck, no question there.

Napa lists just the booster for $81 but I know I can get one cheaper. I'll try to bang that out next weekend.

I see there are lots of adjustments, but I guess I shouldn't have to mess with them just changing the booster.

Thanks guys.
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