Cobra Brake Nightmare - Help :( - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 21 Old 05-01-2009, 06:22 PM Thread Starter
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Cobra Brake Nightmare - Help :(

Hey guys, I could use a little help here... lol. I am having a serious issue with my front left caliper locking up when I stand on the brakes. I have tried 3 master cylinders, 2 boosters, 3 sets of Cobra calipers, and I also tried bleeding the brakes with a pressure bleeder, and also with the conventional foot method. Nothing I do fixes the problem.

The last things I can think of ...

kinked hard line?
rear calipers?

The pressure bleed holds steady at 15psi so I'm fairly confident there are no leaks. The master cylinders were bench bled in a vice and there are no air bubbles coming out from any of the calipers...


82 Mercury Capri - N/A AFR 165, FTI cam 11.99 @ 112
86 LX Coupe - Coyote, T56, 11.55 @ 121
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post #2 of 21 Old 05-01-2009, 07:22 PM
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The only tip I can offer for bleeding stubborn brakes is to rap on each caliper as you're bleeding it with a mallet to dislodge any trapped air bubbles. I can't remember where I picked it up but it can't hurt, right?

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post #3 of 21 Old 05-01-2009, 08:20 PM Thread Starter
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Hey at this point I am willing to try anything... thanks. I'm going to swap the rear calipers here shortly, will report back with results. I'll tap the front and rear calipers with a rubber mallet when I bleed them.

82 Mercury Capri - N/A AFR 165, FTI cam 11.99 @ 112
86 LX Coupe - Coyote, T56, 11.55 @ 121
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post #4 of 21 Old 05-01-2009, 08:21 PM
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What's left inside your factory proportioning valve?
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post #5 of 21 Old 05-01-2009, 09:02 PM Thread Starter
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Quote:
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What's left inside your factory proportioning valve?
Both factory prop valve were gutted per the M2300k instructions.



82 Mercury Capri - N/A AFR 165, FTI cam 11.99 @ 112
86 LX Coupe - Coyote, T56, 11.55 @ 121
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post #6 of 21 Old 05-02-2009, 12:45 AM
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His car has my front calipers rear calipers gutted prop valve and master on it right now. When I had all that stuff on my car it stopped really really good had some track days with it never any problems. We put the rear calipers on so only thing I can think of maybe is a kinked line at this point.. Or is it normal for the left front to lock up first? The only difference from the 2 cars besides the hard line is that I used the factory 5.0 booster and he has a 93 cobra booster.


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post #7 of 21 Old 05-02-2009, 10:33 AM
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Is there any Chance you could get a pressure gage on each wheel (in particular the fronts)? Ideally, you would have two gages so that you could read them at the same time, but I am sure that checking one at a time would also work.

Have you swapped the front soft lines? Is there any chance that the right one could be restricting flow?

Have you talked to Jack?

Good luck.


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post #8 of 21 Old 05-02-2009, 11:44 AM
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thats a good idea chris... Where did you get those braided soft lines from?


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post #9 of 21 Old 05-02-2009, 02:19 PM
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I've had front lines cause similar issues also, they can almost act as a check valve, pressure will push them open, but when the pressure is released, they collapse.

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post #10 of 21 Old 05-02-2009, 06:40 PM
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That's a good reason for brakes to drag, but it doesn't affect the input pressure and therefore doesn't affect one locking up early.

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post #11 of 21 Old 05-02-2009, 07:39 PM
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Mine does the same thing. I have the cobra fronts with rear drums.

Cheaper than VHT and works great.
So sticky it will practically pull your shoes off, just like the movie theater
floor..
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post #12 of 21 Old 05-02-2009, 07:42 PM
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Could something be causing the outside pad to set to close to rotor when the brakes arent beint applied then once you hit the brake there is less air gap in that side possibly. I know i have taken new bearing hubs from different manufactures and set them side by side and they are different heights. Just a thought.

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post #13 of 21 Old 05-02-2009, 08:20 PM
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There's a thread here or cc-com that talks about how it's common for one of the fronts to lock up early, but I thought it was the right side, and I'll be damned if I can find it anyway.
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post #14 of 21 Old 05-02-2009, 08:47 PM Thread Starter
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Quote:
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Could something be causing the outside pad to set to close to rotor when the brakes arent beint applied then once you hit the brake there is less air gap in that side possibly. I know i have taken new bearing hubs from different manufactures and set them side by side and they are different heights. Just a thought.
The car has new Timken hubs on it... which I know many people on this board and over on c-c have used as well. However, I was having the same issue way back when I had untouched 94-95 spindles and Ford hubs, so I'm quite confident that the hubs aren't the problem. I definitely appreciate the insight.

I can't imagine it could be a bad Wilwood valve? I think before anything, my task should be to get a gauge on each caliper and see what the reading is. Will do that tonight (my friend has access to one) and report back.

Very exasperating given that I can drive this car anywhere and the brakes work great around town, yet I've got all these fancy suspension do-da's done and the car is completely not track safe because of the lock-up issue.

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post #15 of 21 Old 05-02-2009, 09:06 PM
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I would recommend checking the corner weights with a scale, if you have not done this yet.
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post #16 of 21 Old 05-02-2009, 11:40 PM
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I agree. I would corner weight the car. Most Fox Mustangs will have about 100lbs of extra weight on the LF tire. With you in the car, adjust any spring perches to get the diagonal weights as equal as possible.

To test the behavior of the brake system, put the front end in the air with a jack. Depress the brake pedal 10 or so times to get rid of any vacuum in the power booster. Place a weight on the brake pedal, that can't fall of or move. Put a torque wrench on the LF wheel stud that is at the top of the wheel. You will need a beam style for this. Put the torque wrench parallel to the ground. Push down to see how much torque it takes to move the wheel. Repeat for the RF side of the car. You may need to adjust the weight that is placed on the brake pedal. If the torque for the left and right side of the car is the same, then you probably have some problem with the corner weights.

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post #17 of 21 Old 05-13-2009, 06:50 PM
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so we did what you said jack with the tq wrench and the passenger side took 20 more ftlbs than the drivers side and the drivers side is the one locking up. doesnt make any sense to me?


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post #18 of 21 Old 05-13-2009, 07:14 PM
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Mat,

How much torque did each side require to start turning the wheel?

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post #19 of 21 Old 05-13-2009, 07:18 PM Thread Starter
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~30 ft/lbs driver's side, ~50 ft/lbs on the passenger side. Done with me sitting in the car maintaining consistent brake pressure, Matt using a Matco beam-type torque wrench and turning the wheels.

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post #20 of 21 Old 05-13-2009, 07:43 PM
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I did it basically how you would set pinion pre load in a rear end constantly turned the wheel and he put the resistance on it we did it several times and each time passenger side was 20 ftlbs more.


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post #21 of 21 Old 05-13-2009, 10:26 PM
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Two concerns here.

1) The wheel must be stopped when you are measuring the amount of torque required to overcome the brake pad friction. The main reason is this. The torque you are applying to the wheel changes constantly if the wheel is turning. This is because the torque that overcomes the friction is that which is applied to the center of the wheel. You are actually applying a torque to one of the wheel studs, which is not concentric with the center of the wheel. That is why I said to put the torque wrench on the top stud, directly above the center of the wheel and keep the torque wrench parallel to the ground. This was just to guarantee that you were applying exactly the same torque to the center of the wheel on both sides of the car. From your description, it wasn't clear if this was done or not.

2) The torque values you measured are so low, that they may be strongly affected by friction (that is not from the brake pads) or something else in the system. The brake torque required to lock the front tire is around 1,200ft-lbs. I realize that you can't make this much torque with the torque wrench, but the higher up you can measure, the more it will tell you.

If you remeasure at a higher pedal force and get torques of 230 and 250ft-lbs, then we know there is probably some friction (that is not in the brake pads) difference between the two sides, since the differential stayed constant. On the other hand if you get torques of 200 and 300ft-lbs, then there is some difference in brake line pressure or brake pad friction, since the differential remained a constant percentage.

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