Foxbody with '98 Cobra brakes, which master cylinder? - Ford Mustang Forums : Corral.net Mustang Forum
 
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post #1 of 28 Old 05-14-2016, 11:09 AM Thread Starter
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Foxbody with '98 Cobra brakes, which master cylinder?

I hate to create a new thread for this but I can't remember the research I did years ago for this: I have '98 cobra brakes on my '92 with a '93 Cobra R booster. I did the brakes many years ago, I _think_ I have the master cylinder from a v6 SN95 on there (1 1/16"?) (Bendix, part numbers 6167C and 4598D appear to be stamped on its face but a quick search hasn't yielded anything). Car originally had the stock '92 booster on it, that is why I think I went with the v6 SN95 unit (per MJbobbit's site, maybe?).

Anyhow, the brakes lock up way too easily but I have just driven around it for years. However, my auto-xer co-driver is pretty tired of worrying about. I need to confirm which master cylinder I have on there but should I be looking at the 1" Cobra unit? Or do I have that backwards and need a bigger bore?

Thanks!

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post #2 of 28 Old 05-14-2016, 08:11 PM
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I have Cobra brakes on mine with the '88 booster and '93 Cobra (1") MC.
The pedal effort might be a little high for some tastes, but modulation is very good. My 130 lb wife drives that car and has not complained about the pedal effort. The pads I have on there are pedestrian "street" pads. A pad with more bite will improve things. Future update list #19,984,875.


'88 LX hatch. Mild 331. Full MM suspension minus the Torque Arm, Cobra Brakes. Not as slow as it used to be.
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post #3 of 28 Old 05-14-2016, 10:32 PM
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I'm running 04 cobra brakes on a 92, a 93 cobra master and stock booster feels good to me. I have run this setup with both stock cobra pads and my current Hawk HP+. I run HPDE and to work and back with it.
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post #4 of 28 Old 05-15-2016, 01:08 PM
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The 93 Cobra MC suits my purposes just fine, kind of firm, not a ton of travel, but easy to modulate.
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post #5 of 28 Old 05-15-2016, 04:16 PM
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When you say the brakes lock up, are we talking about both ends or just the front? If just the front, how close is the rear to locking? Can you get the rear to lock before the fronts (for testing purposes only)?

You need both ends working at max. If they aren't a M/C change may not solve the problem. One of the biggest issues I've had with this sn95 Cobra conversion is getting max rear brake. On my 86 I replicated the 93R system and had hardly any rear brake; even with the prop valve turned full rear max - no lock back there. In my case and with the help of Jack H. we decided the prop valve was defective. Since I've converted to ABS, not certain how the system will work - car isn't yet driveable.
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post #6 of 28 Old 05-15-2016, 10:36 PM Thread Starter
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Are the '93 Cobra m/c and '98 Cobra m/c interchangeable? Both 1"? For some reason, I was thinking the number of brake line ports was different.

Re: bias, I have an biast adjuster. Pretty sure we have video of the rears locking up as well as the front, depending on weight transfer.
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post #7 of 28 Old 05-16-2016, 02:26 PM
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Until the 86 was taken off the road last year, I was also using the 93 Cobra M/C with the stock 86 booster (93R brake replica). I found the pedal feel to be similar to my 01 Cobra with factory pads. The Cobra now has Hawk HPS all around. Big difference in modulation; much easier and responsive than the stock pads. Which made me return to the 86; I've always run the Hawk HPS pads since the 5-lug conversion. The 86's rears aren't doing their share and in comparison to the Cobra, it's easy to tell. BTW, the Cobra likes to use the rears. I generally go through two sets of rears for every front set of pads (why - no idea but the rear pads are tiny). This is a factory ABS car with no mods save for H&R conventional springs, 00R Bils, CAI and Borla cat-back. The Cobra will easily out-brake the 86 without engaging the ABS (threshold is easier to feel with the Hawk pads). I went to ABS on the 86 hoping for better brake performance than what I've had, and to avoid flat-spotting those expensive Rival S tires.
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post #8 of 28 Old 05-16-2016, 06:44 PM Thread Starter
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I forgot to mention, I have a firm, short pedal that takes real finesse. Going from a 1 1/16" M/C to a 1" unit will increase pedal travel a bit, right? That seems to be the consensus for what everyone else is using.
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post #9 of 28 Old 05-16-2016, 07:11 PM
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Quote:
Originally Posted by aurdraco View Post
I forgot to mention, I have a firm, short pedal that takes real finesse. Going from a 1 1/16" M/C to a 1" unit will increase pedal travel a bit, right? That seems to be the consensus for what everyone else is using.
I've wondered just how much of a difference there is when moving up/down 1/16". FWIW, when I did the 5-lug conversion on the 86, I was swapping 73mm SVO calipers and 11" discs to the 38mm twin Cobra PBRs and 13" rotor. Out back I left the 10.25" rotor and 45mm single piston caliper with a fox length 5-lug axle. As noted, Hawk HPS all around. I kept the 1-1/8" SVO M/C. Although I could not get the brakes to lock at all, the pedal was not all that hard, nor that short of a stroke. Honestly, not a whole lot different than with the 73mm SVO caliper.

After consulting with just about anyone who seemed to know their brake stuff, I took their collective advice and swapped to the 93 Cobra 1" piston M/C - and had exactly the same problem.

Turns out that Jack H. had mentioned in passing something about banjo bolt length and if too long would not allow enough fluid even though the bolt was tightened correctly. Pulled the front banjos out and swapped to shorter ones - and got the fronts to lock. Still had rear brake issues which eventually turned out to be a defective prop valve.

The point here is that I didn't feel a bunch of difference between the 1-1/8" SVO M/C and the 93 Cobra (an 1/8" difference), and the smaller M/C didn't solve my problem. I suspect had I swapped banjo bolts earlier, I'd still be running that SVO M/C until the ABS swap.
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post #10 of 28 Old 05-17-2016, 04:42 PM Thread Starter
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I'm hoping Jack will pop in and confirm that I should move to the 1" bore before I pull the trigger.

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post #11 of 28 Old 05-17-2016, 09:29 PM
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Going from the 1.06" to the 1" diameter m/c will decrease the pedal effort by 13%.

1996-2004 brake m/c from a V8 Mustang can only be used on a hydroboost unit. They will not fit on a vacuum brake booster.

On a related note, we recently had a problem with a 1996 Mustang where there wasn't enough rear brake torque. Nothing had been changed on the car that could affect brake torque. The problem turned out to be a gunked up stock proportioning valve. The valve had to be taken apart, cleaned out and reassembled. Note that this was on a car that had its brake fully bled hundreds of times at the track and during testing.

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post #12 of 28 Old 05-17-2016, 10:00 PM Thread Starter
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Hi, Jack, thanks for chiming in.

So, if I have a firm pedal with high engagement now, would decreasing the pedal effort mean that I barely touch the pedal and the brakes lock up? Or would it increase the pedal travel until brake lock up? Or does it just mean it will be easier to modulate; right now, my co-driver probably thinks of my brakes as an on/off switch where "on" often = a quick lock up.

Thanks!
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post #13 of 28 Old 05-18-2016, 03:20 AM
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This is really, really simple.

The brake system between your foot and the caliper piston is just a lever. It is made up of multiple parts. The brake pedal, its pivot point, the m/c, the hydraulic fluid, the brake caliper and the caliper piston. The end result is some leverage ratio between your foot and the caliper piston. Say 20:1. For every 1" that your foot moves down, the caliper piston moves 1/20 of this or 0.05".

Anyone that has used a lever to move something understands the relationships involved. Effort and travel are inversely related with a lever. If you double the length of the lever one the input side, the effort is cut in half, but you need to move the lever twice as far to do the same work. If you halve the length of the lever on the input side, the effort doubles, but you only need to move the lever half as far.

When you change the m/c piston size, you are changing the length of the lever on one side of the fulcrum, so you are changing the effort and travel.

Does your current brake system require too much effort or too much travel to operate? That determines whether you want to make the lever longer or shorter.

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Last edited by Jack Hidley; 05-18-2016 at 03:21 AM.
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post #14 of 28 Old 05-18-2016, 10:15 AM Thread Starter
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I guess too much effort and not enough travel, very hard to modulate. Does that make sense? Thanks!
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post #15 of 28 Old 05-18-2016, 12:33 PM
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Yes, that makes sense. In that case, you need a smaller diameter m/c to make the lever longer to reduce the effort and increase the travel.

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post #16 of 28 Old 05-18-2016, 02:26 PM Thread Starter
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Thanks! Any guess on whether to try 15/16" (94-95 Cobra) vs. 1" (93 Cobra)? I have a 2-port on now and a 5-port gutted stock bias block (2 in, 3 out); either of the above should work, correct?

Thanks!
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post #17 of 28 Old 05-18-2016, 02:40 PM
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Since I haven't driven the car, it is impossible for me to know how much too high the pedal effort is.

As I said above "Going from the 1.06" to the 1" diameter m/c will decrease the pedal effort by 13%."

If you then drop from a 1" to a 15/16" m/c the decrease in pedal effort will be similar. Another 13%.

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post #18 of 28 Old 05-21-2016, 11:07 AM Thread Starter
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Hi Jack,

Thanks, I just figured you would know exactly the pedal pressure and travel from the parts described. I mean, c'mon, what good are you if you don't know that! hehehe j/k

I went ahead and ordered a 1" '93 Cobra unit today. Figured I would start with the smallest increment change and go from there. Seems like a common size for people with similar brake swaps. Thanks, everyone, for the input.

Casey
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post #19 of 28 Old 07-05-2016, 12:41 PM
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Quote:
Originally Posted by aurdraco View Post
Thanks! Any guess on whether to try 15/16" (94-95 Cobra) vs. 1" (93 Cobra)? I have a 2-port on now and a 5-port gutted stock bias block (2 in, 3 out); either of the above should work, correct?

Thanks!

What brakes are out back? 98 cobra up front and cobra (or really any SN95) out back? Do you have the rear drums still?

The 98 cobra brakes are identical to the 94-95 cobra brakes which were vacuum boosted. They used a 15/16" bore MC with the 1995 Cobra R running a 1" bore.

So either of those MC should work if you go by what the factory used as a baseline.


Thing is...if you claim that a GT 1 1/16" MC makes the brakes lock up way to easily, going to an even smaller MC is only going to make the problem worse as the smaller bore means improved leverage at imputing force at the calipers.

IIRC, when I ran my 99+ Cobra brakes front and rear with a 1 1/16", I had the hardest time getting good stopping power from the brakes. It required a LOT of pedal effort. I jumped down to the 1" bore MC, and it was a lot easier to modulate the brakes and get great stopping power.

If you are locking up with a 1 1/16" bore now, you problem will only get worse with the smaller bore MC.

Mike
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post #20 of 28 Old 07-05-2016, 02:18 PM Thread Starter
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98 cobra brakes front and rear.

I understand the smaller M/C will increase leverage and pressure; I am looking for more pedal travel so that modulation is easier. That is how I will avoid locking them up all the time.

Thanks.

Quote:
Originally Posted by Mustang5L5 View Post
What brakes are out back? 98 cobra up front and cobra (or really any SN95) out back? Do you have the rear drums still?

The 98 cobra brakes are identical to the 94-95 cobra brakes which were vacuum boosted. They used a 15/16" bore MC with the 1995 Cobra R running a 1" bore.

So either of those MC should work if you go by what the factory used as a baseline.


Thing is...if you claim that a GT 1 1/16" MC makes the brakes lock up way to easily, going to an even smaller MC is only going to make the problem worse as the smaller bore means improved leverage at imputing force at the calipers.

IIRC, when I ran my 99+ Cobra brakes front and rear with a 1 1/16", I had the hardest time getting good stopping power from the brakes. It required a LOT of pedal effort. I jumped down to the 1" bore MC, and it was a lot easier to modulate the brakes and get great stopping power.

If you are locking up with a 1 1/16" bore now, you problem will only get worse with the smaller bore MC.
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post #21 of 28 Old 07-05-2016, 02:51 PM
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Originally Posted by aurdraco View Post
98 cobra brakes front and rear.

I understand the smaller M/C will increase leverage and pressure; I am looking for more pedal travel so that modulation is easier. That is how I will avoid locking them up all the time.

Thanks.

Gotcha.

In that case, your 1998 setup is identical to the 94-95 setup that used a vacuum booster setup and 15/16" MC from the factory

You can use the 1" MC as well, but pedal effort will be 13% greater

Mike
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post #22 of 28 Old 07-05-2016, 11:59 PM Thread Starter
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You're in Boston? Do you auto-x at Devens in Ayer? Going to any TNiA at Palmer or Thompson?
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post #23 of 28 Old 07-06-2016, 08:50 AM
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Originally Posted by aurdraco View Post
You're in Boston? Do you auto-x at Devens in Ayer? Going to any TNiA at Palmer or Thompson?

I used to AutoX at Devens. Sorta retired these days as kids soak up all my free time (and tire fund). It's been 5 or so years since I've done it.

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post #24 of 28 Old 07-06-2016, 12:41 PM
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Quote:
Originally Posted by aurdraco View Post
98 cobra brakes front and rear.

I understand the smaller M/C will increase leverage and pressure; I am looking for more pedal travel so that modulation is easier. That is how I will avoid locking them up all the time.

Thanks.
You may think that (as I once did, too) but your leg is much better at "feeling" pressure than it is judging distance.
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post #25 of 28 Old 07-07-2016, 08:39 AM
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Maybe you need a different booster, I have 04 cobra brakes on a 92. Kept the stock booster, over boost can make for touchy pedal also. The best cure for over-boosted brakes is a more overlap on your cam. If your car idles too smooth you can get brakes with less feel.

For the record I run a 93 cobra 1" bore master and like the feel of it for road course work. Travel is nice and short, just not good for heel and toe due to pedal placement.
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post #26 of 28 Old 07-07-2016, 08:55 AM
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Quote:
Originally Posted by akylekoz View Post
Maybe you need a different booster, I have 04 cobra brakes on a 92. Kept the stock booster, over boost can make for touchy pedal also. The best cure for over-boosted brakes is a more overlap on your cam. If your car idles too smooth you can get brakes with less feel.

For the record I run a 93 cobra 1" bore master and like the feel of it for road course work. Travel is nice and short, just not good for heel and toe due to pedal placement.
I've got the same setup (99-04 cobra brakes) with the 1" MC and used the 1993 Cobra/SN95 booster.

Brakes aren't that touchy at all with the larger booster. Perfect feel to me.

Mike
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post #27 of 28 Old 07-07-2016, 11:51 PM Thread Starter
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I have a '93 Cobra booster, iirc. The easy lock up, I think, also occurred with the stock '92 Booster which sprung a leak.
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post #28 of 28 Old 07-07-2016, 11:53 PM Thread Starter
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I used to AutoX at Devens. Sorta retired these days as kids soak up all my free time (and tire fund). It's been 5 or so years since I've done it.
What size wheels do you run? I just scored some free Michelin racing slicks (27/65-18) and could probably get more. You'd have to pay to have them mounted but, if you were looking to get back into it, free tires, or very cheap via GT racing tires, usedracingtires.com, etc., is the way to go.
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