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Discussion Starter #1
Hi guys, hopefully, someone can point me in the right directions. We've recently did a 03 cobra swap since we were going to upgrade to SN95 front brakes for a complete 5 lug conversations. I doing this we believe we did all that is recommended by MM which includes the following: 93 cobra MC and Booster, gut stock fox body PV and added end plug, purchased the rear t line splitter from MM as well as the other lines up front that are required.

Fast forward a few months and we finally have her on the road and destroyed the front brakes already as if the rear doesn't work well or very little. So we installed new quality disc's and new pads. She stops amazing but you can tell its really only the front working, can lock up the front brakes easy if I wanted to. Peddle is hard which would make one think that there is no air in the system, right? The FRPP proportional valve is adjusted all the way down so it will achieve the most pressure to the rear, yet still, very little pressure is being applied. Though I didn't install the brake, we just attempted to bleed them again even though the installer stated he bleed them well. Needless to say, we keep attempted to bleed the back rear but there is always air in the brake fluid as we bleed constantly, to the point we have already gone throw 2 bottles and still getting the air out of the back. I've called MM up to ask for any advice, they alone mentioned that it's possible that the MC wasn't benched bleed even though our installed did state he did this. Regardless wouldn't I still be able to bleed the brakes even if the MC wasn't benched bleed?

Any and all help on this would be greatly appreciated!

Regards
Brad
 

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First thing i usually ask is are the rear calipers installed on the correct sides? It is possible to flip them (at least on solid axle) and put them on the wrong sides. The e-brake cable spring should be on the bottom.

Usually a non-bench bled MC gives a soft pedal. I've driven my car with a MC install that wasn't bench bled and had a terribly soft pedal. If your pedal is rock hard with the car off, i wouldn't suspect this.

I've found that if you open the bleeders on the rear calipers a tad bit too much, air bubbles get sucked past the bleeder and into the line making it appear as if the air is coming out of the bleeder hose. You really need to just barely crack the bleeder to where fluid flows.

With the bleeder cracked, how easy is it to pump the brake and get flow. With a full stroke, you should really be able to get a good push and a good amount of fluid to come out of the line.

How were the brakes bled? I've never had good luck with the motive bleeder setup when installing brand new components like new calipers. The pressure bleeding doesn't seem to "scrub" all the air out of the caliper like manually bleeding by pumping the pedal. I usually save the pressure bleeding for simply purging old fluid out with new fluid.

Do you have an IR thermometer? Can you shoot rotor temps and see if the rears aren't being used at all? With my Cobra setup front and rear, after a few good stops the rotors front and rear are pretty similar in temp.

Also...if i read that correctly...you have stock front brakes and the IRS rear brakes?
 

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Discussion Starter #3
Thank you for this response!

First thing i usually ask is are the rear calipers installed on the correct sides? It is possible to flip them (at least on solid axle) and put them on the wrong sides. The e-brake cable spring should be on the bottom.

- No issues in this regards.

Usually a non-bench bled MC gives a soft pedal. I've driven my car with a MC install that wasn't bench bled and had a terribly soft pedal. If your pedal is rock hard with the car off, i wouldn't suspect this.

I've found that if you open the bleeders on the rear calipers a tad bit too much, air bubbles get sucked past the bleeder and into the line making it appear as if the air is coming out of the bleeder hose. You really need to just barely crack the bleeder to where fluid flows.

- This would be interesting if that is the case. I've watched a few videos and one guy shows adding grease around the bleeder so it plugs it up so no air can get around it. Its possible that I do not have any air in the system and its passing through the threads. Wishing there was another way to test, like those speed bleeders but you still need to open up slightly.

With the bleeder cracked, how easy is it to pump the brake and get flow. With a full stroke, you should really be able to get a good push and a good amount of fluid to come out of the line.

- This isn't an issue, it works perfectly other than having air in the fluid all the time.

How were the brakes bled? I've never had good luck with the motive bleeder setup when installing brand new components like new calipers. The pressure bleeding doesn't seem to "scrub" all the air out of the caliper like manually bleeding by pumping the pedal. I usually save the pressure bleeding for simply purging old fluid out with new fluid.

- Originally the shop that installed the IRS and Cobra MC/Booster and lines, as well as the SN95 spindles, used a vacuum bleeder. My daughter and I did the brake push method I've done for years.

Do you have an IR thermometer? Can you shoot rotor temps and see if the rears aren't being used at all? With my Cobra setup front and rear, after a few good stops the rotors front and rear are pretty similar in temp.

- I do have a gun, I should check temps, that's a great idea! I've only gone by the look of them (seeing high/low wear) and also how she's stopping. Fronts are really stopping heaving almost locking up if it hit them too hard, while the rear is very weak.

Also...if i read that correctly...you have stock front brakes and the IRS rear brakes?

- SN95 GT's spindles and brakes not cobra's, sadly my rears are almost as big as my fronts lol. Will do cobra 13" upgrade eventually. These though should be much better than the foxbody stockers.

- To recap, if my brakes are actually fully bleed correctly and the air I was seeing is from the fitting no in the system. I'm thinking my issues are valving or that damn FRPP inline valve not allowing enough pressure to go to the backs. I've read that those valves do not let much pressure to the backs even at full max pressure (all the way turned in). Wishing I knew more about this since I'm pointing this to be my issue.
'
 

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I think the mismatched brakes might have to do with it as well. Fox's are pretty light, and don't need much brake on the rear. The front does most of the work and when normal street driving, the rear brakes don't really need to do much. Road course/AutoX...different story.

I forget the specs on the valves as it's been some time, but even full out, there's a reduction of 90% to what the front line pressure is with the valve fully open, down to 55% or so when you screw it all the way in. And the valve really adjusts knee point.

I forget where mine is set. I think i was conservative and have it set mostly reduced as my front brakes (13" rotors/C6 calipers) are more than adequate for my spirited street cruising. But i used the IR thermometer for initial "tuning" in addition to some controlled stops in a lot to see if rears would lock up.
 

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Saying that the car has SN95 GT front brakes on it doesn't tell us anything useful. There are two very different brake systems used on the front of SN95 GT Mustangs. 1994-98 and 1999-2004. You need to specify which one.

98% of the time, we tell customers to not use a 1993 Mustang Cobra brake booster. This isn't your problem, but it won't make the brakes better.

If there is air coming out of the bleeder hose all of the time, then there is almost definitely air getting in past the threads. When opening the bleeder screw, only do it 1/4 turn. Any more than this may let air in.

Speed Bleeders are a waste of money. The bleeder screw must be closed when you you release the brake pedal. If it isn't, either air or thread sealant is going to get sucked back into the brake system. Either is bad.

A properly functioning (nonbroken) proportioning valve has absolutely no affect on fluid flow below its breakpoint pressure. When the FRPP valve is set to full rear brakes, the breakpoint pressure is around 900psi. When you are bleeding the brakes, the pressure in the system is near zero, because the blleeder screw is open. It is impossible for the proportioning valve to have any affect.
 

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Discussion Starter #6
Saying that the car has SN95 GT front brakes on it doesn't tell us anything useful. There are two very different brake systems used on the front of SN95 GT Mustangs. 1994-98 and 1999-2004. You need to specify which one.

98% of the time, we tell customers to not use a 1993 Mustang Cobra brake booster. This isn't your problem, but it won't make the brakes better.

If there is air coming out of the bleeder hose all of the time, then there is almost definitely air getting in past the threads. When opening the bleeder screw, only do it 1/4 turn. Any more than this may let air in.

Speed Bleeders are a waste of money. The bleeder screw must be closed when you you release the brake pedal. If it isn't, either air or thread sealant is going to get sucked back into the brake system. Either is bad.

A properly functioning (nonbroken) proportioning valve has absolutely no affect on fluid flow below its breakpoint pressure. When the FRPP valve is set to full rear brakes, the breakpoint pressure is around 900psi. When you are bleeding the brakes, the pressure in the system is near zero, because the blleeder screw is open. It is impossible for the proportioning valve to have any affect.

Thank you for your reply. You are correct the calipers are different as well as the spindles between those years. Thought the disc size were all the same though (gt's).

Agree on speed bleeders, hopefully my issue was the air going around the threads while bleeding.

Regarding the FRPP valve. Wondering if you're missing what I'm thinking i'm after understanding. I believe the valve isn't allowing as much pressure as my system may need to the rear even at full open. Believe these are not a 1 to 1 ratio even at full open.
 

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The SN95 front brake rotors are all the same from 1994 to 2004, however they have very little affect on the brake bias.

The amount of torque created by the caliper clamping the brake rotor is a function of the distance that the center of the pad is from the center of rotor. This is a nearly linear relationship. A 10% change in rotor diameter would create about a 7% increase in brake torque and therefore brake bias.

If you increase the diameter of the piston in the brake caliper by 10%, there is a 21% increase in brake torque and therefore brake bias. This is because the clamping force from the caliper is a function of the piston AREA, NOT diameter.

Changes in the piston diameters in a brake system almost always completely dominate the brake balance of the brake system.

The difference in front caliper piston area between the early and late SN95 GT front calipers is 17.8%. This will create a huge change in brake bias.

With regard to proportioning valves.

Up to the break point pressure, they absolutely no affect on the system. Above the break point pressure, they reduce the output pressure RISE at a given percentage of the INCREASE in input pressure. This behavior is much easier to understand by looking at a graph of their transfer function.

https://wilhelmraceworks.com/images/blog/brake_pv/wilwood_pv.jpg

With valve on your car fully open, when you apply enough pressure to the brake pedal to lock up the front brakes, there is almost no chance that you have even reached the break point pressure of the valve, so there is no chance that it is affecting the rear brakes at all.
 

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Discussion Starter #8
Hi Jack, this is good, I love the comprehension of this as well as a better understanding of what is going on. If what I'm understanding is correct, you are saying that that valve only changes its "breaking point" once it reaches a certain pressure? Wow, I've thought this all wrong, I thought it just varies the pressure applied to it linearly.. as pressure goes up it gives a percentage less. This is strange if this is true than the valve really isn't doing anything at all.

Regarding the front brakes calipers, honest I do not know. Can I share a photo with you to determine what they are? Regarding the piston size related to my issue of not having enough rear bias pressure I want. Hopefully, there is something I can do about this since I'd love to have the 4 tires almost lock up at the same time.
 

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No, the valve never changes the breakpoint pressure itself.

The break point pressure is the thing that is being adjusted, when one turns the knob or moves the lever on the proportioning valve.

In the image that I linked above shows the behavior of the proportioning valve when the break point pressure is adjusted to one of seven different settings. All of the curves are overlaid on one graph. When the proportioning valve is set to 100psi:

If you apply 100psi of brake fluid pressure to the inlet of the valve, there will be 100psi at the output port. If you then apply 200psi to the inlet port, you get 150psi at the outlet port. If you apply 300psi to the inlet port, you get about 190psi at the outlet port. So on and so forth.

If you set the breakpoint pressure to be 650psi:

And apply 700psi to the inlet port, you get about 680psi to the outlet port. If you apply 800psi to the inlet port, you will get 730psi to the outlet port.

When the breakpoint pressure is set to 100psi, that will be when the knob is set to minimum rear brake. When the valve is set to 900psi, that will be when the knob is set to maximum rear brake.

The job of the proportioning valve is to take a system which is parabolic (nonlinear) and try to emulate this behavior with only linear elements. If you look at post 1 and 12 of the link below, it gives an overview of this.

https://forum.miata.net/vb/showthread.php?t=333680

Regarding the front brake calipers. If the caliper body is steel, they are the 1994-98 models. If they are aluminum, then they are the 1999-2004 models. You can try to stick a magnet to them.
 

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Discussion Starter #10
Hi Jack, I spent some time re-reading and searching to understand the setup a little better. Believe I comprehend what is said to be correct in the setup as well as I'd like to share my recent results after bleeding the back brakes. Today I went for a cruise and attempted to lock up the brakes again to see how much front brake different there is. Sure enough its exactly the same, front dives and lock up very easily. Measure temp of disc's after driving. Fronts are 900 degrees while the backs are 230 degrees. Now to figure out why and have to get more pressure to the back since they are barely working... BTW the fronts are alum calipers, magnet doesn't stick and it looks like alum not cast steel.

Any help would be greatly appreciated.
 

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If the calipers are aluminum, then you have the 1999-2004 GT/V6 calipers with dual 43mm pistons.

With the rear brakes installed on your car, the brakes should have a normal brake balance with them installed. This means that if your adjustable proportioning valve is turned all the way to full rear brakes, that when the car is stopped hard, it should spin from the rear brake lockup.

You don't necessarily have too little pressure to the rear brakes. The cf (coefficient of friction) of the pad material has a large effect on the friction created, which determines the resulting brake torque. If the cf is low on the pads at one end of the car, the brakes will do much less work at that end and the rotor temperature will be much lower.

I would start by purchasing and installing the exact same brand and compound of front pads in the rear of the car. This will ensure that the cf of the front and rear pads are the same.

When you bled the rear brakes, did you still get air bubles out of the bleeder or did that stop?

The differential pressure valve in the rear of the combination valve might be sticking, which could restrict pressure to the rear brakes. I would remove the nut on the back of the valve which faces the firewall. There is a valve body inside. Remove this. Clean out the inside of the combination valve body really well with a bottle brush and brake cleaner. Clean the valve assembly. Coat it in brake fluid, Reinstall it. Rebleed the system.

The image below shows a cross section of the combination valve from a similar car.

http://cliff.hostkansas.com/images/2009/cvalve_name.jpg
 

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I would also check the part number on the steering arm of the spindles. If you have 1999-2004 calipers, you probably also have 1999-2004 spindles. Using these on a 1988 Mustang with a stock k-member, will create an uncorrectable bumpsteer condition. You need to use 1994-95 spindles. See the link below for more information.

https://www.maximummotorsports.com/tech_front_susp_spindle_warning.aspx
 

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Discussion Starter #13
I would also check the part number on the steering arm of the spindles. If you have 1999-2004 calipers, you probably also have 1999-2004 spindles. Using these on a 1988 Mustang with a stock k-member, will create an uncorrectable bumpsteer condition. You need to use 1994-95 spindles. See the link below for more information.

https://www.maximummotorsports.com/tech_front_susp_spindle_warning.aspx
Did my research with this one prior, I wanted the 94-95 spindles since that don't stick out as far ;-) They also seem to be a little harder to come by these days.

If the calipers are aluminum, then you have the 1999-2004 GT/V6 calipers with dual 43mm pistons.

With the rear brakes installed on your car, the brakes should have a normal brake balance with them installed. This means that if your adjustable proportioning valve is turned all the way to full rear brakes, that when the car is stopped hard, it should spin from the rear brake lockup.

You don't necessarily have too little pressure to the rear brakes. The cf (coefficient of friction) of the pad material has a large effect on the friction created, which determines the resulting brake torque. If the cf is low on the pads at one end of the car, the brakes will do much less work at that end and the rotor temperature will be much lower.

I would start by purchasing and installing the exact same brand and compound of front pads in the rear of the car. This will ensure that the cf of the front and rear pads are the same.

When you bled the rear brakes, did you still get air bubles out of the bleeder or did that stop?

The differential pressure valve in the rear of the combination valve might be sticking, which could restrict pressure to the rear brakes. I would remove the nut on the back of the valve which faces the firewall. There is a valve body inside. Remove this. Clean out the inside of the combination valve body really well with a bottle brush and brake cleaner. Clean the valve assembly. Coat it in brake fluid, Reinstall it. Rebleed the system.

The image below shows a cross section of the combination valve from a similar car.

http://cliff.hostkansas.com/images/2009/cvalve_name.jpg
Good thinking the valve that should be easy to take apart.

Regarding bleeding. Again I do not know if they are actual bubbles in the system for coming around the threads. Going by the pedal feel its a rock which makes me believe it's coming around the threads. If there is another way of telling I would do it.

Regarding the pads, all the same as is the rotors from the same brand/type.
 

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Doesn't really help you much here, but i was curious. I took my car for a ride last night and brought my IR thermometer. I didn't go crazy getting brake temps up, just a few hard stops to get some heat into the rotors. I got out and shot myrotors. 200F up front and 180ish in the rear. That's with 13" front and 11.65" rear. Motorcraft pads out back and AC delco up front (because i'm running C6 calipers).

Again, doesn't really help you since my setup is completely different, but just an FYI.
 

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Thanks Mustang5L5, thank you for this. Yes that seems more about where they should be. I know in my heart that my fronts are working much harder than the rears are.
 

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