Brakes Frustration and Options

[Mbufford]

Well, I encountered my first infuriating moment of my build.

I spent yesterday evening painting my calipers for the 2015 IRS 11.65 brakes. They turned out excellent, and I was excited to install them this evening. Until I downloaded the instructions…

When the first page tells me to cut another ear off my knuckles, which I painted before fully assembling my complete IRS over the weekend, and use a bracket to adapt the brakes… because they’re apparently not 2015 IRS brakes at all, but were instead adapted for use with 15” rims. Well, I have 17” rims, and only ordered the “2015 IRS Brakes” because it’s reasonable to assume that something marketed as such is actually 20 freaking 15 brakes!

So now, I have to set this crap aside and buy another set of rear brakes. And, meanwhile, I sincerely doubt FFR would accept a return since I painted them, if they would at all anyway…

I’m so furious right now that I don’t want to touch the damn car for a while.

[gbranham]

Yeah, definitely need to read all the instructions before you start wrenching. I've learned that lesson several times because I didn't read ahead.

[maclonchas]

Well I would hold off on doing anything for a day or two and contact Kleiner, Forte, Levy or Everson and see what they recommend as an alternate solution. They have a wealth of wisdom on these cars and know the brake solutions. I ended up using these for my rear brakes (https://thefactoryfiveforum.com/showthread.php?39218-2015-IRS-Brake-Installation-Frustration).

I would also echo gbranham on prep before each area doing work. I built folders on my computer and stockpiled pictures and threads that came from other builds for each task so I could get an idea of what I might be facing before I tackled a task.
Cheers,

Bill

[Mbufford]

Reading the instructions has nothing to do with my problem.

My problem is that FFR misrepresented what they sold me and others. The order form clearly says “2015 Mustang IRS 11.65” Rear Brakes.” If you buy something advertised as “2015 Mustang” parts, it is reasonable to expect to receive 2015 Mustang parts. These are neither 2015 NOR Mustang parts.

Had they not misrepresented what this is, I would have gladly spent the extra $150 to get the actual 2015 Mustang part. They know damn well that what they’re selling is not 2015 or Mustang parts.

Are you suggesting that I should have read the entire set of instructions before ordering my kit? That would be an absurd expectation. After a search of both forums, I’m not the first person to end up in this situation.

I’ll be calling FFr today to give them an opportunity to make this right.

[rich grsc]

Why do you think these won't work?

[Mbufford]

It’s not a matter of whether they’ll work, it’s a matter of whether I got what I believed I was purchasing: 2015 Mustang brakes that match and fit my 2015 Mustang suspension.

As I mentioned in a previous post—had I known these were not 2015 Mustang brakes, I would have bought the actual 2015 Mustang brakes.

I’ve already painted my knuckles and fully installed my IRS. I’m not ripping that back apart and cutting my knuckles to install brakes that I should never have bought, much less to fit 15” rims that I’ll never have on my car.

[RoadRacer]

Respectfully - (really) - everything should be test installed before things start getting painted.

Painting before even downloading the installation instructions just added so much risk to the build process. It's been said before, but you have to read ahead in the manual (preferably to the end), before you start. It's like reading these threads here before you build.. you learn new and better ways.

[Jeff Kleiner]

Well, I encountered my first infuriating moment of my build.

I spent yesterday evening painting my calipers for the 2015 IRS 11.65 brakes. They turned out excellent, and I was excited to install them this evening. Until I downloaded the instructions…

When the first page tells me to cut another ear off my knuckles, which I painted before fully assembling my complete IRS over the weekend, and use a bracket to adapt the brakes… because they’re apparently not 2015 IRS brakes at all, but were instead adapted for use with 15” rims. Well, I have 17” rims, and only ordered the “2015 IRS Brakes” because it’s reasonable to assume that something marketed as such is actually 20 freaking 15 brakes!

So now, I have to set this crap aside and buy another set of rear brakes. And, meanwhile, I sincerely doubt FFR would accept a return since I painted them, if they would at all anyway…

I’m so furious right now that I don’t want to touch the damn car for a while.

You had two options for IRS rear brakes on the order form:

16765 - 2015 Mustang IRS 11.65” Rear Brakes....................... +$699.00

16237 - 2015 Mustang IRS 13” Rear Brake Kit........................ +$850.00

Or you could have chosen neither and sourced OEM Mustang brakes yourself.

Like Road Racer I say this respectfully; it seems that you made your choice without being fully informed. With that said though I'll also say that
FFR should have confirmed your brake selection and made sure that you were aware of the differences when you had your order review.

Jeff

[OB6]

Putting instructions and build approach aside for a moment, I think the broader point is that the options on FFR's order form are not very clear. Both options would imply that they are indeed 2015+ Mustang brake components, when in fact that's only true of the 13" option.

193243

[Hoooper]

The 2015 mustang did not come with 11.65" rear brakes, it would have been impossible to supply you with an 11.65" OEM 2015 mustang brake setup

[michael everson]

https://go.skimresources.com/?id=130832X1596018&isjs=1&jv=15.4.2-stackpath&sref=https%3A%2F%2Fwww.ffcars.com%2Fthreads%2Froad ster-order-questions.645870%2F%23post-6159359&url=https%3A%2F%2Fwww.ebay.com%2Fitm%2F36371888089 6&xs=1&xtz=300&xuuid=e7fa4308fd6e2ef0e91434de9de2eaef&xjsf=other_click__auxclick%20%5B2%5D
Buy these and move on. Direct bolt on. I have bought many sets from him.
Mike

[OB6]

https://go.skimresources.com/?id=130832X1596018&isjs=1&jv=15.4.2-stackpath&sref=https%3A%2F%2Fwww.ffcars.com%2Fthreads%2Froad ster-order-questions.645870%2F%23post-6159359&url=https%3A%2F%2Fwww.ebay.com%2Fitm%2F36371888089 6&xs=1&xtz=300&xuuid=e7fa4308fd6e2ef0e91434de9de2eaef&xjsf=other_click__auxclick%20%5B2%5D
Buy these and move on. Direct bolt on. I have bought many sets from him.
Mike

Definitely the way to go... they're new take-offs. I bought the same from someone instead of sourcing through FFR.

[Mbufford]

How would I know that? I’m not a Mustang guy…

All’s well though. I spoke with Dan at FFR, who was very reasonable. I’ll strip the paint and ship back for a refund, and then I’ll grab the set that Mike and Terry commented on.

I tested some brake cleaner for stripping the paint, and it came right off. So, note for the future: Dupli-Color Caliper Paint comes off with brake cleaner… kind of surprised with that—I figured it’d be engineered to stand up to brake cleaners. Maybe because it’s not fully cured?… ��*♂️

[scrubs]

Bruh, if you are pissed already with the build..you better have your doc prescribe you some nitroglycerine pills. FFR screws up tolerances, order of build, etc.

Before each step I would suggest a quick forum search as it changed my plan several times.

Chillax, first world problems, eh?

j

[ggunter]

I paid a couple stupid taxes on my build too. Mostly out of ignorance, and some out of excitement to get moving on the build. We all live and learn.

[Buzzsaw]

I bought Ford 13” brakes ( calipers and rotors ) from Mike Forte. When they arrived they weren’t Ford brakes and were after market products. Mike said Ford no longer makes the 2015-2017 brakes and after market is all that’s available. Looking at Summit and other sites you can’t buy Ford breaks because they show out of stock. The brake assembly fit easily, I did tell Mike his web site was misleading and he agreed and is in the process of changing it.
All of us “Newby” builders should heed Scrubs comments in my month and a half of building there has been some reservations of “ is that how it’s supposed to go together. If you stress to much about the build it will take all the fun out of it!

[Mbufford]

Interesting, that’s the first I’ve heard that about Forte’s kits, and that Ford stopped producing them. My first thought was “well why would they still be making and selling the 8.8 diffs.” Then I realized how stupid that thought was.

I bought the brake kit Mike linked to above, which are new take-offs, for a very reasonable price, and they’ll be here Monday. I also jumped on Rock Auto and got the ebrake cables for $45 plus shipping and taxes. Overall, I’m getting actual Mustang brakes and cables for roughly half the price FFR charges for the T-bird brakes.

I would just say, for the record, that I really appreciate FFR’s willingness to work with me on resolving this issue. I’ll be out a little for the cost of shipping them back—but the mark of successful negotiation and compromise is where both parties walk away less than thrilled at the result.

[michael everson]

MBufford. Don’t forget 4 bolts that are not included in the brake package.
https://www.mcmaster.com/product/98093A815

[Mbufford]

I was thinking about this in the shower not 15 minutes ago and asked myself, I wonder if the build book says what hardware I need.

Thanks again, Mike!

[BRRT]

The rear caliper mounting bolts are found at auto parts stores, if you want to avoid shipping from McMaster.
I got mine at O'Reilly.
https://www.oreillyauto.com/detail/c/brakebest-hardware-hydraulics/brakebest-disc-brake-caliper-bracket-mounting-bolt/bhh0/h869/v/a/146463/automotive-car-2019-ford-mustang?q=brake+caliper+bolts&pos=4

I stole this from LMR:
https://thefactoryfiveforum.com/attachment.php?attachmentid=193304&d=1702655559

[Mbufford]

Excellent resource, thank you!

[CraigS]

You will be very satisfied w/ the OE take-off brakes. I bought my rear suspension parts as a complete takeout unit from a salvage yard which included the brakes. They work so much better than the old Fox GT brakes I had previously. And pretty much all brake pad manufacturers make pads for a 15 Mustang. OTOH I am constantly surprised how often FFR screws up specs or listings like the brakes quoted above lacking pertinent info which leads to misunderstandings. A single line of text added to each listing could make the differences crystal clear.

[totem]

Well, I encountered my first infuriating moment of my build.

I spent yesterday evening painting my calipers for the 2015 IRS 11.65 brakes. They turned out excellent, and I was excited to install them this evening. Until I downloaded the instructions…

When the first page tells me to cut another ear off my knuckles, which I painted before fully assembling my complete IRS over the weekend, and use a bracket to adapt the brakes… because they’re apparently not 2015 IRS brakes at all, but were instead adapted for use with 15” rims. Well, I have 17” rims, and only ordered the “2015 IRS Brakes” because it’s reasonable to assume that something marketed as such is actually 20 freaking 15 brakes!

So now, I have to set this crap aside and buy another set of rear brakes. And, meanwhile, I sincerely doubt FFR would accept a return since I painted them, if they would at all anyway…

I’m so furious right now that I don’t want to touch the damn car for a while.

What wrong? The 11.65in brake that fits the 15in wheels will also fit the 17in wheels.

[Mbufford]

If you read the rest of the posts, you’ll see my issue wasn’t particularly whether they would work.

[Mbufford]

https://go.skimresources.com/?id=130832X1596018&isjs=1&jv=15.4.2-stackpath&sref=https%3A%2F%2Fwww.ffcars.com%2Fthreads%2Froad ster-order-questions.645870%2F%23post-6159359&url=https%3A%2F%2Fwww.ebay.com%2Fitm%2F36371888089 6&xs=1&xtz=300&xuuid=e7fa4308fd6e2ef0e91434de9de2eaef&xjsf=other_click__auxclick%20%5B2%5D
Buy these and move on. Direct bolt on. I have bought many sets from him.
Mike

Thanks again for this recommendation.

I got the take off set bolted on yesterday, no fuss and fit perfectly. Do you use the bracket on the soft brake line that comes with them?

Note for anyone who will buy from this source in the future. The calipers didn’t come with a full set of pads (not that you’d want the factory pads anyway). So, having some ordered/bought and ready to go when your calipers arrive would be a good idea. I hadn’t even thought about pads, so I had to do that after getting the brakes in.

[michael everson]

Strange. Mine all came with pads. 10 sets so far.
Mike

[Mbufford]

Interesting. Guess I may have been the one-off. Either way, I would have replaced the factory pads anyway.

[OB6]

Mine came with pads and I kept them because they only had 300 miles on them. Factory pads are typically very good. Akebono is usually my first choice for aftermarket street pads.

[Mbufford]

Yeah, no doubt factory ford pads are good. But pads that have been on a car, taken off and handled, no telling what oils, solvents, etc., they’ve come in contact with. Replacement pads is cheap insurance IMO.

[OB6]

Yeah, no doubt factory ford pads are good. But pads that have been on a car, taken off and handled, no telling what oils, solvents, etc., they’ve come in contact with. Replacement pads is cheap insurance IMO.

Oh definitely. I only kept mine because they were barely used.

[CraigS]

Don't forget that the OE pads were specced for a car w/ power brakes. So they may not work real well w/ manual brakes.

[OB6]

Don't forget that the OE pads were specced for a car w/ power brakes. So they may not work real well w/ manual brakes.

Very good point, but I figured I'd keep them until I have a chance to test, which is a ways out.

[Mbufford]

Interesting. With that in mind—are any of the aftermarket pads for 2015+ mustangs engineered for use with non-power brakes?

[Inman Lanier]

That's not quite a valid question - there are NO cars made w/o power brakes anymore. Having said that, however - power assist or not the pads have nothing to do with 'compatibility'. The pressure in the lines that clamps the pads (via the calipers) is what matters. I'm assuming you're a Mark IV and I don't know how FFR designs that build (I'm an old Mark 1) - but assuming there is a proper ratio of pedal length to pin placement (where the MC attaches to the pedal) - you're fine. What they used to do with manual brakes is have a different moment arm on the brake pedal so that your manual action had a higher force at the MC pin. All the booster does on power brakes is add more force (as if you're pushing much harder). Because of that, the pin location is designed so that your foot has less force on the MC pin.

I'd be careful playing with aggressive rear pads unless you have an adjustable proportioning valve. If you have too aggressive a rear pad, you'll lock up the rears in the rain and possibly swap ends. No bueno.

That being said - pads are all about friction coefficient and temperature of use. I would recommend you build your car with the cheapest rear pads you can get a hold of (maybe someone here on the forum will donate a set to match your backing plate design) - try out the brakes and see what the balance is. If you find your rear brakes are inadequate, then you can start either dialing up more pressure back there (via your proportioning valve if you have one), or go with a higher friction coefficient pad at that time. The converse is also true - if you don't have an adjustable proportioning valve and your rears are too aggressive - install said valve and dial down the rear pressure until the balance is safe and to your desires.

[Mbufford]

My question was more theoretical in response to Craig’s comment above. I’m boosting my system and using ABS, so it doesn’t apply to my build. I’m more just trying to expand my knowledge of such things.

In a way, you did answer the question though: it would seem that the higher grip pads, like Hawk HP and such, would be a better fit for non-boosted brake; while something cheaper like Duralast Gold or equivalent would be more appropriate for boosted brakes.

In terms of the proportioning valve, what are your thoughts on using one with ABS. As someone with no prior knowledge of braking systems, it would seem logical to me that ABS wouldn’t need one since the modulation stops the lockups. But, it would also seem useful to use proportioning to get bias set as well as possible to prevent needless ABS corrections every time you brake. Am I too far off base there?

Thanks for the wisdom!

[Dgc333]

FYI, brake pads have a huge impact on how well you can stop your car. Brake pads are rated by their coefficient of friction at low temperature and high temperature. On the street you want a pad that has a high coefficient at low temperatures and on the track you want one with a coefficient of friction at high temperatures. All pads are marked with two letters that represent the coefficient of friction. The first letter being low temp and the second letter the high temp rating. Typical parts store pads will have a low temp rating of D, E or F and the same rating for high temp. High performance street pads are typically F or G for both ratings. Track pads will almost always have a higher high temp rating than the low temp rating.

If you can get pads that are rated GG you will have about the most aggressive street pads you can get. Note: these tend to wear faster, through more dust on the wheels and wear out the rotors faster.

In order to get maximum friction from your pads they have to be broken in properly. The coefficient of friction between the pad and the iron of the rotor is relatively low. But by bedding pad material into the surface of the rotor during break in you will realize maximum performance.

[Inman Lanier]

In a way, you did answer the question though: it would seem that the higher grip pads, like Hawk HP and such, would be a better fit for non-boosted brake; while something cheaper like Duralast Gold or equivalent would be more appropriate for boosted brakes.

In terms of the proportioning valve, what are your thoughts on using one with ABS. As someone with no prior knowledge of braking systems, it would seem logical to me that ABS wouldn’t need one since the modulation stops the lockups. But, it would also seem useful to use proportioning to get bias set as well as possible to prevent needless ABS corrections every time you brake. Am I too far off base there?



Apologies for the wordy response - trying to cover all bases here...

Let's deal with these different issues. Forget how boosted brakes has anything to do with your pad needs. it all comes down to whether or not the force that is imparted on the MC piston is high enough to generate enough pressure in your system to lock each wheel, or not. Non-boosted brakes simply use a higher force multiplier in the pedal to pin arrangement. With boosters, there's a different mechanical advantage in the pedal (lower) and the booster takes your system vacuum times the booster area to add force to the MC piston. If setup correctly by the manual brakes, the same force is imparted on the MC piston so that you generate enough pressure in your calipers to lock any/all of the wheels.

To the best of my knowledge, ABS systems do not modulate pressure, it simply cycles a given brake line on/off rapidly to have a similar effect - but ABS only does it in the event something aberrant happened and any wheel(s) have locked up. ABS stopping distances have only now gotten close to max brake torque without lockup (my aged brain remembers 12.5% slippage is the most effective tire stopping). Regarding proportioning valves, I've never actually checked any of my ABS cars to see - but essentially all cars come with a proportioning valve before ABS - as a fixed pressure reduction. Rear brakes have significantly lower pressure than fronts because of weight transfer and the physics of stopping (internet says 30% - 40% - I think 40% is on the high side). Stepped diameter master cylinders are costly (different pressures each port), so the simple proportioning block with a spring and valve seat does a fixed reduction of pressure to the rear. I'm guessing (but have never checked) that ABS systems have them too for the simple physics of stopping the cars. The ABS system shouldn't even know or sense there's a pressure reduction to the rear calipers. BUT - I've never checked so that's my engineering guess there.

Cobras like ours weigh in the vicinity of 2200 - 2300 lbs. The brakes that are on our cars are FAR MORE THAN NEEDED to stop these cars (I'm not talking the extremes of 800 HP on a short race track with no cooling). This is especially true with boosted brake systems designed for cars over 50% more than our weight.

Having said that, OEM street pads are far more than adequate for these cars especially if you have a power booster. My Mark 1 (they were un-boosted) was originally inadequate; I had to use an exotic pad with a friction coefficient of ~.55 up front as well as a higher than normal rear pad friction coefficient. Since then, I bought the Whitby retrofit booster kit. Even with the larger diameter MC (which means you need MORE force by the leg or booster to get enough line pressure) - it was WAAAYYYY too much brake for my car. I then got a vacuum regulator so I could reduce the vacuum assist and (dial down the force added by the booster) to where the brakes didn't throw me through the windshield when I touched them.

Now that I know you're boosted - don't worry about pad compound. Your brakes have WAY more torque capability than this car needs. Yes, I recommend you get a proportioning valve for the rear because like Ford Motorsports did (wrong) with their first retro-kit for Fox body Mustang rears - your balance may be off (I don't know). Of course it's likely easy to add later as long as you pick a spot in the rear brake line run where you'd want to put it in advance and allow for it. The pressure going to the rear brakes is almost never too low, so with a proportioning valve, you can dial the balance to your taste.

Find out from someone else who has your combination of master cylinder diameter, booster, front and rear calipers and see what they run. If you don't mind the extra expense, yes, I recommend you use a proportioning valve in the rear circuit (it's nothing other than a pressure regulator that adjusts your rear pressure lower). But, it may not be needed.

A relevant story is my 1967 Cougar that came with 4 wheel drums, manual. No booster. One could lock up the tires with no issues. It stopped well, but if you wanted any kind of aggressive driving the shoes would fade. I retrofit the factory front disks and power booster that were the option on this car and the Mustangs. With the smallest rear cylinders (least rear brake action), there was way too much rear bias (stock proportioning block ineffective). My fronts are a stock compound (semi-metallic as I recall). I bought a proportioning valve, and cranked to all the way reduced pressure in the rear, still too much. I finally put rear disks in (kinda wanted to anyway). Unfortunately the kit used calipers designed for a Cadillac Seville (heavier car), so they're still biased just a bit too much. It's better - OK in the dry, but in the wet the rears tend to lock up. I don't want that. My solution now is to get a more aggressive front pad. I'm getting EBS yellow based on the cost, friction coefficient, and temperature range. They will basically act like a stock front pad with higher torque to improve the balance (friction coefficient ~0.42 and up vs. stock likely 0.36). If you wish to PM me, I have data regarding pads and temperature ranges for Hawk, Carbotech and EBC. I'd be happy to share it with you.

[Mbufford]

Wow, thanks for the detailed response!

I wouldn’t say money is no object in my build. But anything that can save me any heartache or time after I’m on the road is something I’ll pay for.

With the ABS going in the front (next to where the Breeze battery box mounts), I think I would need to put the valve just forward of the foot box wall, or right by the ABS unit itself, since that is the only stretch where rear brakes will have a common supply line.

Do you (or anyone else) recommend any particular valve from any particular source/manufacturer?

Thanks again,

[i.e.427]

Matt, reach out to me via email. Some of the info you are receiving here, regardless of how detailed, is going to get you or someone else hurt.

Frank

[Dgc333]

FWIW, if you have the Wilwood pedal box with the dual master cylinders you do not need a proportioning valve. This setup has a balance bar between the two master cylinders that allows for front to rear bias to be adjusted by turning the bar. You can get a remote adjustment kit from Wilwood that allows you to adjust the bias from the driver's seat while driving.

[CraigS]

I would stay away from a valve as Dgc333 says. The problem w/ a valve is that it is a pressure limiter. Say you put one in the rear line and your max braking works great in dry good traction conditions because the valve keeps the rears from locking up. What happens in the wet when you can't brake as hard? The valve never comes into action because the pressure doesn't get high enough. Use the built-in bias adjuster. Here is friction info for Hawk Motorsports compounds.
https://www.hawkperformance.com/compounds/motorsports
Here is info for a couple of street compounds. You have to click each individually.
https://www.hawkperformance.com/market/street-car
I have experience w/ the HPS, HPS5.0 and HP+. Their friction level goes up in the order I listed them. The two street compounds are very low dust. The HP+ is like magic but it does dust a lot. I use this to help w/ dust.
https://www.armorall.com/product/outlast-brake-dust-repellent/
I also bought a tiny $50 shop vac w/ a brush hose end that I dedicated to cleaning wheels.
Notes on the graphs. For our street driving and autocross look at the 100F part of the graph because that is where your pads will be working. We want as close to a horizontal line as we can get so the braking doesn't change. IE the red and purple lines in the motorsports chart are compounds we would never want. Also read the comments for each compound. There are a couple of other Motorsports compounds that might work looking at the graphs and the comments may offer some info on which to choose. Until I did the IRS retrofit and got the 15 Mustang rear calipers I was always trying for more rear brake. I ran HPS on the front and HP+ on the rear for many years. A note on MC sizes; As mentioned pedal leverage ratios (applicable to the old Mustang pedal box but not the Wilwood) as well as hydraulic ratios (comparing the area of the MC piston to the area of the caliper piston) have an effect on how a person's pedal pressure is transferred to the caliper. The problem w/ using more ratio to reduce pedal effort is that it also increases pedal travel. You could have two setups w/ the same braking performance; high ratio but harder pads vs lower ratio w/ grippier pads. Most people would prefer the second one because the pedal would not need to move as far. RE; ABS. As mentioned above ABS is a limiter, all it can do is shut off a caliper for an instant and then let it work for an instant. (For this discussion forget stability and or traction control systems) The HUGE advantage is that it can do that for just one wheel at a time as needed. Which means you can slam the brake pedal for all you are worth and it will let you keep steering to avoid an object. Instead of the front wheels immediately locking, the brakes will pulse, and if you turn the steering, the brakes will pulse differently on the left and right sides. BUT, the basic system still needs to be tuned w/o the ABS just like back in the day because it is quite rare that we slam the pedal as hard as we can.

[rich grsc]

Wow, thanks for the detailed response!

I wouldn’t say money is no object in my build. But anything that can save me any heartache or time after I’m on the road is something I’ll pay for.

With the ABS going in the front (next to where the Breeze battery box mounts), I think I would need to put the valve just forward of the foot box wall, or right by the ABS unit itself, since that is the only stretch where rear brakes will have a common supply line.

Do you (or anyone else) recommend any particular valve from any particular source/manufacturer?

Thanks again,

DON"T.. Thats all I can say.

[Inman Lanier]

Matt, reach out to me via email. Some of the info you are receiving here, regardless of how detailed, is going to get you or someone else hurt.

Frank

Unfortunate but true. Some uninformed opinions here on this post.

[Inman Lanier]

FWIW, if you have the Wilwood pedal box with the dual master cylinders you do not need a proportioning valve. This setup has a balance bar between the two master cylinders that allows for front to rear bias to be adjusted by turning the bar. You can get a remote adjustment kit from Wilwood that allows you to adjust the bias from the driver's seat while driving.

If you have this configuration, that's the best option.

[Inman Lanier]

FWIW, if you have the Wilwood pedal box with the dual master cylinders you do not need a proportioning valve. This setup has a balance bar between the two master cylinders that allows for front to rear bias to be adjusted by turning the bar. You can get a remote adjustment kit from Wilwood that allows you to adjust the bias from the driver's seat while driving.

Agreed - this would be your best option for brake bias adjustment.

[i.e.427]

Unfortunate but true. Some uninformed opinions here on this post.

Particularly the one that stated you want less rear brakes...........................

[Inman Lanier]

Particularly the one that stated you want less rear brakes...........................

If you're referring to me, perhaps you misread my statements. First off, with boosted brakes, none of these cars will ever have too low a line pressure to lock up any of the wheels.

I also stated that you want a minimum brake torque at each wheel to be able to lock up any of the wheels. If you read back you'll see that. Once balanced - If you put more aggressive pads in the rear at that time, you can potentially swap ends. Locking up the fronts before the rears is rarely a dangerous situation. The converse usually is a dangerous situation. You should never play with brake torque at either end until you've had a chance to assess both ends working together (assess the bias/balance). But yes, assuming you have sufficient brake torque, the safe direction in rear brakes lacking any other data is less vs. more.

As I stated, I don't know the Mark IV configuration that is recommended, but I would be willing to bet it's a collection of brake pieces from other cars (not specifically designed for this application). The mere statement someone made that you can select different rear disk sizes from FFR shows a variation in brake torque to the rear. The same caliper on larger rotors will effect a higher brake torque to the rear because the moment arm of the braking (i.e. centroid of the pads where they clamp) is at a larger radius. Furthermore, no one has yet even mentioned effective caliper piston area. That's the other fundamental part of the equation - piston area of each caliper a directly proportional to the brake torque that wheel generates. A 15-20% increase in rear wheel brake torque can be a dramatic difference.

The physics of braking are immutable.