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Thread: New brake problem on my 93 SC

  1. #1
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    New brake problem on my 93 SC

    When doing burnouts at the track this weekend the brake pedal got very hard when I suddenly got off the throttle and onto the brakes to get the car stopped before the staging light box. For the first second or so it required about 5 times the normal pedal effort to get the car stopped, then the pedal softened and brakes started working like normal. Didn't have any problem on the top end of the track, probably because I don't get on the brakes quickly.

    I'm assuming this has something to do with not having enough vacuum going to the booster, or possibly the booster check valve not functioning properly. Any pointers from you guys that are familiar with this brake system on what to check or replace would be apreciated.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  2. #2
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    I had something similar happen to mine about 3 years ago when I was doing allot of auto-x, happened a couple of times never really did find out what it was but I replaced the check valve for the heck of it and it has never happen since. You could check it and see.

  3. #3
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    Are you using line locks? If so a solenoid may be sticking.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  4. #4
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    Quote Originally Posted by Mike Puckett View Post
    Are you using line locks? If so a solenoid may be sticking.
    Mike,

    Nope...the brake system is stock except for the 13" Cobra brakes on the front. This car never has had very strong brakes but they were always good enough that it felt safe, until this started happening.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  5. #5
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    Quote Originally Posted by Flip View Post
    I had something similar happen to mine about 3 years ago when I was doing allot of auto-x, happened a couple of times never really did find out what it was but I replaced the check valve for the heck of it and it has never happen since. You could check it and see.
    I agree that sounds like the easiest place to start.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  6. #6
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    Quote Originally Posted by David Neibert View Post
    When doing burnouts at the track this weekend the brake pedal got very hard when I suddenly got off the throttle and onto the brakes to get the car stopped before the staging light box. For the first second or so it required about 5 times the normal pedal effort to get the car stopped, then the pedal softened and brakes started working like normal. Didn't have any problem on the top end of the track, probably because I don't get on the brakes quickly.

    I'm assuming this has something to do with not having enough vacuum going to the booster, or possibly the booster check valve not functioning properly. Any pointers from you guys that are familiar with this brake system on what to check or replace would be apreciated.

    David
    Sounds like you are on the right track to me. Would have to be the booster or the check valve to the reservoir. Do you have a larger reservoir installed due to the power adder? You could always pull vacuum on the booster and check it.

  7. #7
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    Quote Originally Posted by frdlvr30 View Post
    Sounds like you are on the right track to me. Would have to be the booster or the check valve to the reservoir. Do you have a larger reservoir installed due to the power adder? You could always pull vacuum on the booster and check it.
    I was talking about this with Rod earlier today and he asked me about the vacuum reserve tank that's supposed to be up in the passenger side fender. There is no tank there and it was most likely removed by those jokers in Dayton who originally built the turbo system. Those guys didn't relocate stuff that was in the way, they just tossed it.

    So in addition to the check valve I'll be looking for a larger than stock vacuum tank. Looks like I've got plenty of room to mount one behind the driver's side of the front bumper cover.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  8. #8
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    Quote Originally Posted by David Neibert View Post
    I was talking about this with Rod earlier today and he asked me about the vacuum reserve tank that's supposed to be up in the passenger side fender. There is no tank there and it was most likely removed by those jokers in Dayton who originally built the turbo system. Those guys didn't relocate stuff that was in the way, they just tossed it.

    So in addition to the check valve I'll be looking for a larger than stock vacuum tank. Looks like I've got plenty of room to mount one behind the driver's side of the front bumper cover.

    David
    The never ending story, huh? Thats you problem David. Especially in situations like that where you are going from WOT to braking immediately. The booster will never hold enough vacuum within itself for situations like that. Plumb yourself a reservoir with a check valve to manifold vacuum and it will be solved...How did the car do otherwise? Other than Im guessing, some traction issues.....!!!
    Edit- I read your post on the other thread. Man that thing will fly!!! I like how you accidentally went 11.2 at 100mph (Obviously seriously pedaling it!!) Is that car getting any kind of twisting damage??
    Last edited by frdlvr30; 07-22-2009 at 12:30 AM.

  9. #9
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    Quote Originally Posted by frdlvr30 View Post
    The never ending story, huh? Thats you problem David. Especially in situations like that where you are going from WOT to braking immediately. The booster will never hold enough vacuum within itself for situations like that. Plumb yourself a reservoir with a check valve to manifold vacuum and it will be solved...How did the car do otherwise? Other than Im guessing, some traction issues.....!!!
    Edit- I read your post on the other thread. Man that thing will fly!!! I like how you accidentally went 11.2 at 100mph (Obviously seriously pedaling it!!) Is that car getting any kind of twisting damage??
    The car did fine, but it's still a little slow getting into full boost when launching from a dead stop. In the past I was able to get as much as 12 pounds of boost on the trans brake, but now I'm only getting about 5. Some of that is probably because I disconnected the electronic boost controller which was programmed to hold the wastegate closed until it reached 14 psi. I plumbed in a manual controller that doesn't have that ability, but it doesn't do any of the goofy stuff the electronic controller did either, so I like it better. On the street it easily spins the tires from a stop without any boost and once the rpms get over 3500 and the boost starts hitting hard it instantly pegs the tach. On the track (with drag radials) it doesn't spin the tires off the line, but does spin them on the top end of 1st gear.

    I think I can get it to do a better job of launching by tweaking the tune a little (leaner and more timing advance) and raising the rpms a couple hundred on the 2 step rev limiter. Once I turn the boost up, I think I'm going to need some 28" or 29" slicks to stay off the 6800 rpm rev limiter on the top end of 3rd . It really depends on how much converter slip I'm going to get.

    At this point I'm launching fairly soft (1.7 60ft), so there really isn't any twisting and the stock halfshafts seem to be holding up fine. Probably will start having some problems if I can get it to start pulling 1.5 60fts, but I'll cross that bridge when I come to it.

    Back to the brakes....do you are anyone else here have a vacuum diagram showing how the vacuum canister and brake booster are supposed to be plumbed on a 93 SC ?

    David
    Last edited by David Neibert; 07-22-2009 at 11:11 AM.
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  10. #10
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    Quote Originally Posted by David Neibert View Post
    The car did fine, but it's still a little slow getting into full boost when launching from a dead stop. In the past I was able to get as much as 12 pounds of boost on the trans brake, but now I'm only getting about 5. Some of that is probably because I disconnected the electronic boost controller which was programmed to hold the wastegate closed until it reached 14 psi. I plumbed in a manual controller that doesn't have that ability, but it doesn't do any of the goofy stuff the electronic controller did either, so I like it better. On the street it easily spins the tires from a stop without any boost and once the rpms get over 3500 and the boost starts hitting hard it instantly pegs the tach. On the track (with drag radials) it doesn't spin the tires off the line, but does spin them on the top end of 1st gear.

    I think I can get it to do a better job of launching by tweaking the tune a little (leaner and more timing advance) and raising the rpms a couple hundred on the 2 step rev limiter. Once I turn the boost up, I think I'm going to need some 28" or 29" slicks to stay off the 6800 rpm rev limiter on the top end of 3rd . It really depends on how much converter slip I'm going to get.

    At this point I'm launching fairly soft (1.7 60ft), so there really isn't any twisting and the stock halfshafts seem to be holding up fine. Probably will start having some problems if I can get it to start pulling 1.5 60fts, but I'll cross that bridge when I come to it.

    Back to the brakes....do you are anyone else here have a vacuum diagram showing how the vacuum canister and brake booster are supposed to be plumbed on a 93 SC ?

    David

    Dave what canister are you talking about the only canister I know the car has is for the cruise control. I had to make myself a generic one when i had a larger then adequate camshaft. I am unaware there is a factory canister for the brakes.

  11. #11
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    I am unaware there is a factory canister for the brakes.
    Ditto...I'm just going by what I was told. Anyone else care to confirm or deny the existince of a vacuum canister on a 93 SC ?

    David
    Last edited by David Neibert; 07-22-2009 at 12:31 PM.
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  12. #12
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    Quote Originally Posted by David Neibert View Post
    Ditto...I'm just going by what I was told. Anyone else care to confirm or deny the existince of a vacuum canister on a 93 SC ?

    David
    I have never found one on my 93 and there is no reference to one in the factory service manual.

  13. #13
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    Quote Originally Posted by Flip View Post
    I have never found one on my 93 and there is no reference to one in the factory service manual.
    Okay thanks for clearing that up.

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  14. #14
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    Quote Originally Posted by ricardoa1 View Post
    Dave what canister are you talking about the only canister I know the car has is for the cruise control. I had to make myself a generic one when i had a larger then adequate camshaft. I am unaware there is a factory canister for the brakes.
    Ricardo,

    How much better did your brakes work after adding the vacuum canister ?

    David
    1991 SC AOD 4.2..2.3 Whipple..........10.910 @ 125.61
    2016 SRT Challenger Hellcat.............10.597 @ 135.78

    My Garage

  15. #15
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    Marginal, it was not a fix but a slight band aid. It helped at times but if I used up all of it like going down an incline with the brakes applied I would loose them near the end of the incline. Scary.
    Get a big enough canister to handle it but problem I found is placement.

    93+ ABS brakes IMO suck although much more reliable. I think the earlier cars offers better brakes even with the small rotors.

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