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  1. #1

    Default Brake Booster - 85 GT

    Hey all,

    I'll get right to the point. My brakes SUCK! A lot!!

    Before I get in to the problem, a little bit about my car. It's a 351 swap and there is a LOT of stuff under the hood that is not connected to anything! That being said, the car runs and drives great (brakes excluded) and all of my electronics (gauges, lights etc...) operate as they should.

    Now, to the issue. My brakes are ridiculously bad! A week or two ago, I found a fair amount of brake fluid on my driveway. Looked like it was coming from where the Master Cylinder connects to the Brake Booster. I opened the reservoir and the front bowl was basically empty! Either there was a leak or the guys at Jiffy Lube didn't secure the cap correctly when they checked all the fluids during my oil change.

    Before I noticed the fluid on the driveway, they were bad but I don't recall them being almost non existent. Anyways, I went through and bled the brakes and basically cycled new fluid all the way through the system. Buttoned everything back up and they still suck. For shoots and giggles, I will be swapping out the Master Cylinder this weekend and then I will bleed the brakes again (I will bench bleed the Master first) but I'm curious about something? Last night as I was surveying the situation, I noticed that the Vacuum line from my booster doesn't appear to be hooked up to anything (see pic)!

    Now, I'm not Mr. GoodWrench or anything but mechanically speaking, I know enough to be dangerous. Lol! That being said, my knowledge of the Brake Booster is somewhat limited. I would say that it's a safe bet that the Vacuum line doesn't have any effect on my (potential) leak but I would imagine that it plays some sort of role in my brakes sucking as bad as they do. Thoughts? Thanks!

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    Last edited by dagassa; 08-28-2018 at 10:48 AM.

  2. #2
    FEP Senior Member 83gt351w's Avatar
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    It’s possible the MC is bad, as you said it appeared to be leaking. I’d bleed it on the booster, that’s the best way to do it. As for your vacuum lines, it sounds like you’re trying to run a bare bones Mustang, that you’re standard stuff works, like your defrost, floor heat, etc. On my 351, I run my booster vacuum hose to the intake vacuum port. My hose to operate my heater box runs to the carb, and my distributer runs to the front port on the carb, with my pcv runs to the carb back. That’s all the lines I have. Make sure to use VACUUM hose, and not fuel line. If the booster isn’t hooked up correctly, pedal effort will be really high. Hope this helps.

  3. #3

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    Quote Originally Posted by 83gt351w View Post
    It’s possible the MC is bad, as you said it appeared to be leaking. I’d bleed it on the booster, that’s the best way to do it. As for your vacuum lines, it sounds like you’re trying to run a bare bones Mustang, that you’re standard stuff works, like your defrost, floor heat, etc. On my 351, I run my booster vacuum hose to the intake vacuum port. My hose to operate my heater box runs to the carb, and my distributer runs to the front port on the carb, with my pcv runs to the carb back. That’s all the lines I have. Make sure to use VACUUM hose, and not fuel line. If the booster isn’t hooked up correctly, pedal effort will be really high. Hope this helps.
    Thanks for your response! Much appreciated!!

    The previous owner of my car did the 351 swap and I'm slowly going through everything trying to finish all the stuff that was left unfinished. My distributor and carb are setup just like yours so I think we're good there!

    Regarding the Booster Vacuum line, I considered your comment and then took another look at things and the distribution tree is in fact connected to the vacuum port on the manifold so the booster is pulling from there. I just didn't see it because because it's on the bottom and on the other side from where I was taking the pic.

    I'm hopeful that the MC is the culprit here. It would make sense as I had almost no pedal pressure. It goes straight to the floor!

  4. #4
    FEP Senior Member
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    It sounds like your master cylinder could be leaking and fluid is probably getting into the booster. Replacing the master cylinder will fix the master cylinder, but if you get brake fluid into the booster it will soon destroy that as well. After you remove the master cylinder check the inside of the booster the best you can for signs of brake fluid. If you see any you should definitely replace the booster. Even if you don't see fluid in there it might not be a bad idea to replace it anyway while you have the m/c off.

    Your booster vac line is connected to a vacuum manifold. Just because the port on the other side of the booster hose is plugged doesn't mean the booster isn't seeing vacuum. Is any part of the manifold connected to vacuum anywhere? I would suspect yes, but I cannot see the entire manifold and where everything goes.
    '89 XR-7 5 Speed
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  5. #5

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    Quote Originally Posted by KevinVarnes View Post
    It sounds like your master cylinder could be leaking and fluid is probably getting into the booster. Replacing the master cylinder will fix the master cylinder, but if you get brake fluid into the booster it will soon destroy that as well. After you remove the master cylinder check the inside of the booster the best you can for signs of brake fluid. If you see any you should definitely replace the booster. Even if you don't see fluid in there it might not be a bad idea to replace it anyway while you have the m/c off.

    Your booster vac line is connected to a vacuum manifold. Just because the port on the other side of the booster hose is plugged doesn't mean the booster isn't seeing vacuum. Is any part of the manifold connected to vacuum anywhere? I would suspect yes, but I cannot see the entire manifold and where everything goes.
    I will definitely be sure to check the booster as well while I have the MC off. From the angle I took the pic at, I didn't see that the manifold vacuum line is in fact attached to the distribution tree. It's on the other side on the bottom so I didn't see it initially. I had assumed originally (as you stated) that the booster line would be attached to the port directly opposite where it was hooked up. That's what I get for assuming! Lol!

    Thanks!

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