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Discussion Starter #1
Thanks for the help guys. Here's what I got. I got an 88 GT with SN-95 5-lug Cobra upgrade with rear 11 3/4 cobra disc conversion. I installed Russel front and rear SS lines, 95 GT booster and master cylendar, adj prop valve, 3-2 conversion, and Cobra rear brackets. I have a really, really hard brake pedal, and the brakes aren't working very well. I measured my vaccume and I have 10 lbs at idle, and if you rev it it will climb up to 20 lbs max. The brakes don't work well any time, so I am leaning towards a bad brake booster, but I wanted to make sure that 10 lbs of vaccume is enough to operate the system sufficiently, or 20 lbs enough? Thanks guys
 

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Most people have about 17-20lbs to make it workable.

Did you check for any vacuum leaks?

Your booster may not be bad at 10lbs, it may not get enough.
 

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i use mine with about 10-12" of vacuum. full cobra brakes, 2000V-6 booster, 95GT master cylinder. pedal is firm but works great

adam
 

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Discussion Starter #4
I'm running a pretty stout Ed Curtus Camshaft in this 347. I have noticed that when you hit the brakes once in a while, you can here a slight chirp coming from under the dash. Maybe the booster is leaking when the pedal is compressed. I think I might try another booster, before spending money for an electric vaccume pump.
Adam, did you have any trouble bleeding your cobra calipers front and rear. ALso, where do you have your prop. valve adjusted at for your final adjustment.
Thanks again Matt. Yes, I have checked for vaccume leaks. None to be found. Just a really big hydraulic roller from Ed. Any other ideas?
 

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your booster is supposed to make a little noise. it vents the vacuum in the booster....

no problem bleeding at all. it helps a bit if you tip the rear calipers up a bit so that all the air gets out. im not sure exactly where i have my prop valve adjusted. just trial and error

take care,
adam
 

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Discussion Starter #6
Matt, instead of running a vaccume pump, could I run a vacume canister like alot of the older 80's street cars did. Or, could I at least try something like this to see if thats def. my problem or not. Thanks,
 

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bad88t-top said:
Matt, instead of running a vaccume pump, could I run a vacume canister like alot of the older 80's street cars did. Or, could I at least try something like this to see if thats def. my problem or not. Thanks,
THere is already one on the car. It is in the passenger side fender.

You can always add another one, very easy to find vacuum balls at the wrecking yards.

You can also go down in MC bore size to the 93 cobra 1" unit. Same threads as the 1 1/16" you are running now.
 

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Discussion Starter #8
Matt, I just got off the phone w/ Baer, and he recommended a 94-95 Cobra master cylendar due to its 15/16 bore. Will the threads be the same as what I have now? I greatly appreciate it. I think thats the route I'm going to try first, the master cylendar change. Letm eknow BIll
 

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Discussion Starter #9
Allright guys, Matt, Adam, and, and Dugger, Last night I installed a 93 Cobra master cylendar on my conversion. It made the pedal feel better, but still poor braking as you get to the bottom of the pedal. I can put all my weight on the brake pedal, but still can't get the brakes to lock up. The pedal feels good up high, but the lower it goes, towards the bottom its very hard with poor braking. Since the 93 Cobra master cylendar made a difference, do you think that I should try the 94/95 Cobra 15/16 master cylendar. Lastly, for your first adjustment, do you adjust the proportioning valve 4 turns in or 4 turns out for your starting point. Thanks guys,

Once again, I'm running 95 Cobra front brake Kit, 95 Cobra rear brake kit, SS lines, 93 Cobra master, and 95 GT booster all on 88 GT w/ 10" of vaccume. Thanks again
 

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sounds almost like you have air in the lines

adam
 

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Discussion Starter #11
ONe thing I didn't think of when bleeding the rear lines last night was to tilt the calipers. Think that makes a difference?
 

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Discussion Starter #12
Well, no luck w/ the 93 Cobra master cylendar either. I checked the vaccume on my buddy's 95 cobra with cam intake, etc... He's pulling 12.5" of vaccume without any braking problems. I'm pulling 10.5"-11" of vaccume. I'm thinking that its not a vaccume issue anymore, and a part issue. I guess I'll try swapping the booster and we'll see what happens. Does anyone know if there is a difference between the 95 Cobra and GT booster, and if so, do you think that could be my problem. Let me know, please. Thanks
 

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I have a 95GT master and booster.. I see about 11in vac in gear(its not lbs in vacuum btw, its inches, boost is pounds) and it stops wonderfully. COuld be booster, check valve, vac lines, etc.
 

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Discussion Starter #14
Thanks adam. I'm going to pick up another booster today and try it. I'm hoping that that's the problem. If not, I'm going to just keep bleeding them. Thanks again everyone. I'll let you know monday what happens.
 

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FWIW I have cobra brakes and because i'm buildig a fairly nasty cammed 351 I called ford racing to see what the vacume requirments were for the big cobra booster and they said they would not recomend less at 15 inches of vacume unless I had a vacume resivour. Ive operated this brakes with no booster control (engine off coasting downhill, don't ask :) ) and they are monsters.
 

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Discussion Starter #16
Thanks for the info man. How can people such as my friend w/ 12.5" of vaccume and Adam D. w/ 11" of vaccume be operating fine then? I have talked w/ several people about my vaccume problem,and they are all running less than 15" of vaccume. I actually read a book last night, and it said that the average amount of vaccume is 10" of vaccume, and that you will never see more than 15" of vaccume on a vehicle. Thanks,
 

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I get 15-23" of vacuum while driving, depending on if i am decelerating, and how much throttle, etc...and around 10-15" at idle
 

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Discussion Starter #18
Well guys, I finally fixed my brake problem. I ended up getting a defective booster. I replaced it on Saturday with another one and it fixed my problem. Thanks for all the help guys. Later on
 
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