Brakes FOX BRAKES ISSUES

PonyGTrider · Jan 11, 2026

Hi all,

A question to all of you.
On my ‘90 GT I have the five lug/brake conversion upgrade from a 2001 bullitt but have been dealing with not too effective braking issues. I have the 95 booster and master cylinder installed for this upgrade. For all of this project I gutted the stock proportioning valve and installed the WIlwood adjustable proportioning valve by the driver’s side strut tower.

The problem like I said is not having a strong braking and I noticed my rear brake pads are almost new while the front ones have just a quarter of the pad material left. This is telling me that the rear is not braking hardly anything.

The wilwood adjustable proportioning valve has ten turns from all closed (Counter clockwise) to full open (Clockwise). I adjusted it eight turns clockwise and still not having good rear brake pressure bias. what should I do? Should I completely delete the stock proportioning valve??? My rear brakes are not working properly even adjusting the adjustable proportioning valve eight out of ten turns clockwise?
Any help will be greatly appreciated!

Mustang5L5 · Jan 12, 2026

Put the rear end of the car on jackstands and put it in gear. If you barely apply any brake pedal, do the rear wheels stop? They should.

When you bleed the brakes, do you get solid flow from the rear bleeders, or a trickle. This will be the most telling sign here. If you get strong flow per pump, then you should take a look at the calipers themselves.

Also, MC is not ideal. The 94-95 GT MC (1 1/16" bore) is a tad too big for 99-04 Cobra brakes. Pedal is likely soft with a long stroke. A 1993 Cobra MC is closer to ideal with the larger SN95 booster, but I don't think this is the cause of your main issue.

PonyGTrider · Jan 12, 2026

Mustang5L5 said:
Put the rear end of the car on jackstands and put it in gear. If you barely apply any brake pedal, do the rear wheels stop? They should.

When you bleed the brakes, do you get solid flow from the rear bleeders, or a trickle. This will be the most telling sign here. If you get strong flow per pump, then you should take a look at the calipers themselves.

Also, MC is not ideal. The 94-95 GT MC (1 1/16" bore) is a tad too big for 99-04 Cobra brakes. Pedal is likely soft with a long stroke. A 1993 Cobra MC is closer to ideal with the larger SN95 booster, but I don't think this is the cause of your main issue.

Even though I have replaced all the brake fluid and don’t seem to have any air in the system I will follow your advice.

I tried to follow the procedure by Wilwood on how to set the rear brake bias I still have not achieve that solid braking I was expecting.
Thank you

Mustang5L5 · Jan 12, 2026

When you bled the brakes, you did it with the valve fully open? What sort of fluid flow do you have?

One common issue i've seen is that if the brake fluid on these cars hasn't been flushed periodically and is old and full of water, is that the combo/proportioning valve gets rust inside that blocks one of the ports. I've seen it cause issues with rear braking. That's why im asking questions regarding fluid flow to the rear when you crack a bleeder valve. If flow is a trickle or requires a lot of force, then this may be the case.

AeroCoupe · Jan 12, 2026

I run a 1995 MC and booster with Cobra brakes on my Coupe and can tell you that if you have bled the brakes properly and everything is working as it should the car should just about throw you through the windshield if you are not belted in under hard braking. I have the 93 Cobra MC and booster on my 83 Thunderbird with Cobra brakes and it's the same way.

I am with Mustang5L5 in that you need to make sure you bench bled the MC properly and that it is not bypassing, the calipers are installed correctly (bleeders at the highest point when installed), and the system was bled properly.

So a Wilwood manual proportioning valve has a maximum decrease of 57% of the system pressure so 43% of the pressure will still go to the rear brakes with it pinched down as far as it will go. The reason for stating this is even with it pinched all the way down the rear brakes will work.

When you say that you gutted the stock combination valve (OEM brass block that al the brake lines go to) did you remove the plug on the front of it along with the piston and spring that were behind that plug and then replace the plug with a solid one? There is no need to mess with the rear plug as that is the shuttle valve.

Mustang5L5 · Jan 12, 2026

Ah that's a good point about bench bleeding. Air in an MC tends to get trapped at the end because it's the highest point on an installed MC. The front portion is also the portion of the MC which sends fluid to the rear brakes.

PonyGTrider · Jan 12, 2026

AeroCoupe said:
I run a 1995 MC and booster with Cobra brakes on my Coupe and can tell you that if you have bled the brakes properly and everything is working as it should the car should just about throw you through the windshield if you are not belted in under hard braking. I have the 93 Cobra MC and booster on my 83 Thunderbird with Cobra brakes and it's the same way.

I am with Mustang5L5 in that you need to make sure you bench bled the MC properly and that it is not bypassing, the calipers are installed correctly (bleeders at the highest point when installed), and the system was bled properly.

So a Wilwood manual proportioning valve has a maximum decrease of 57% of the system pressure so 43% of the pressure will still go to the rear brakes with it pinched down as far as it will go. The reason for stating this is even with it pinched all the way down the rear brakes will work.

When you say that you gutted the stock combination valve (OEM brass block that al the brake lines go to) did you remove the plug on the front of it along with the piston and spring that were behind that plug and then replace the plug with a solid one? There is no need to mess with the rear plug as that is the shuttle valve.

Yes that braking efficiency is what I was expecting after the whole conversion but I’m not there yet.
I totally agree with what you’re saying about the percentage of residual pressure even with the valve fully closed.

I absolutely removed all the guts from the stock proportioning valve; piston spring and everything.
And I’m positive I did the MC bench bleeding procedure so I’m a bit confused on what’s going on.
I think my best bid will be to start from scratch removing the MC and do everything from the beginning.

Thank you

PonyGTrider · Jan 12, 2026

Mustang5L5 said:
Ah that's a good point about bench bleeding. Air in an MC tends to get trapped at the end because it's the highest point on an installed MC. The front portion is also the portion of the MC which sends fluid to the rear brakes.

I agree too, I will remove the MC and perform that step again

Thank you

PonyGTrider · Jan 12, 2026

Mustang5L5 said:
When you bled the brakes, you did it with the valve fully open? Unscrewed all the way out? What sort of fluid flow do you have?

One common issue i've seen is that if the brake fluid on these cars hasn't been flushed periodically and is old and full of water, is that the combo/proportioning valve gets rust inside that blocks one of the ports. I've seen it cause issues with rear braking. That's why im asking questions regarding fluid flow to the rear when you crack a bleeder valve. If flow is a trickle or requires a lot of force, then this may be the case.

I don’t think I misunderstood the instructions, it says that for full pressure or fully open the valve’s knob must be fully in turning it clockwise…
“Pressure is reduced to a maximum of 57% of inlet pressure by turning the knob counter-clockwise (all the way out). For full line pressure, turn the knob clockwise (all the way in)”

I bleed the rear brakes with the knob all the way in (fully open)

To tell you the truth I didn’t see the flow at the bleeders because I’m using the one person bleeders (Easy bleeding) on all calipers so I haven’t physically seeing how much they flow.

I flushed the system not that long ago and refilled it with Preston synthetic fluid so there shouldn’t be that much moisture or rust in there.

Thank you

Mustang5L5 · Jan 12, 2026

PonyGTrider said:
I don’t think I misunderstood the instructions, it says that for full pressure or fully open the valve’s knob must be fully in turning it clockwise…
“Pressure is reduced to a maximum of 57% of inlet pressure by turning the knob counter-clockwise (all the way out). For full line pressure, turn the knob clockwise (all the way in)”

I'm sorry. I am mistaken. You are right. Full rear pressure is turning the valve fully in.

PonyGTrider · Jan 12, 2026

Mustang5L5 said:
I'm sorry. I am mistaken. You are right. Full rear pressure is turning the valve fully in.

Not a problem!
Having agreed on that, how the hell my brakes are not as solid as they should be if I almost fully opened the valve with eight out of ten turns clockwise from from fully closed to fully open? Beyond my understanding!
Again I will start from scratch by removing the MC and performing the bench bleeding procedure, reinstalling it and replacing the whole fluid while bleeding all corners.

Thank you

AeroCoupe · Jan 12, 2026

Do you have a soft pedal? With the car off push on the pedal. If it moves more than a little bit then stops you have air in the system. If it slowly goes the the floor and if you pump it and it gets solid then you have air in the system. If it goes to the floor no matter how many times you pump it the MC is bad. At least that is how I have always diagnosed this stuff.

PonyGTrider · Jan 13, 2026

AeroCoupe said:
Do you have a soft pedal? With the car off push on the pedal. If it moves more than a little bit then stops you have air in the system. If it slowly goes the the floor and if you pump it and it gets solid then you have air in the system. If it goes to the floor no matter how many times you pump it the MC is bad. At least that is how I have always diagnosed this stuff.

The pedal doesn’t feel spongy just don’t feel the pads biting the disc hard enough.
I will test the pedal as you suggested later on because right now I have MC out of the car and will do a bench bleeding procedure and see how it feels later.

Thank you

86HO5.0 · Jan 13, 2026

OP--What's your complete brake setup?

This isn't the first time I've read about a SN95 swap that the rear brakes are essentially non-functional on.

PonyGTrider · Jan 13, 2026

Not using SN95…
Here’s my complete set up:
I bought a new kit of 2001 Bullitt special edition calipers from the formerly Ford Racing Performance.
Front stainless steel braided lines, 95 brake booster and master cylinder, Prestone synthetic brake fluid, Willwood adjustable proportioning valve, one person easy bleeders, ceramic brake pads, and Centric brake discs.

Right now I’m getting all the fittings double flaring lines to make my custom stock proportioning valve delete.
My master cylinder is on the bench waiting to be bench bled, will verify the length of the brake booster pushrod, put everything back together and bleed the whole system.

As you all can see I’m paying attention to every single detail and hope for the best, We’ll see…

AeroCoupe · Jan 13, 2026

Okay, just so you know you do have a SN95 brake conversion (94-04 is SN95). It appears you are running the front 13" rotors and calipers from a 01 Bullitt but what are you using in the rear? Fox drum, 94-04 V6/GT, or Cobra/Bullitt?

PonyGTrider · Jan 13, 2026

Progress: I just measured the brake booster pushrod/master cylinder air gap and found it to be perfect at 0.010”.
Now I will continue with my custom stock proportioning valve delete.
Before I bench bleed my ‘95 GT MC should I go for the ‘93 cobra smaller bore diameter? I’m reading mixed comments about this preference

AeroCoupe · Jan 13, 2026

Before we can answer that please answer my questions above.

PonyGTrider · Jan 13, 2026

AeroCoupe said:
Okay, just so you know you do have a SN95 brake conversion (94-04 is SN95). It appears you are running the front 13" rotors and calipers from a 01 Bullitt but what are you using in the rear? Fox drum, 94-04 V6/GT, or Cobra/Bullitt?

I have the full 01 Bullitt kit, front and rear…

Mustang5L5 · Jan 13, 2026

PonyGTrider said:
Before I bench bleed my ‘95 GT MC should I go for the ‘93 cobra smaller bore diameter? I’m reading mixed comments about this preference

I think you can solve your issue here and then make that decision later on.

When I ran the 95 GT MC with my cobra brakes, my pedal was soft, but all 4 brakes worked nicely. But it was more pedal travel than I wanted.

Given that you said that your pedal feels ok, proceed for now, however keep this in the back of your mind. One reason the pedal feels ok now is if you have a restriction in flow, it builds pressure at the MC. This could be related to your issue. Solve the rear caliper issue first, and then if you determine you don't like the pedal feel, change the MC later.

Brakes FOX BRAKES ISSUES

PonyGTrider

Mustang5L5

That is…until I whipped out my Bissell

PonyGTrider

Mustang5L5

That is…until I whipped out my Bissell

AeroCoupe

lube between the nut and the face. I know my lubes

Mustang5L5

That is…until I whipped out my Bissell

PonyGTrider

PonyGTrider

PonyGTrider

Mustang5L5

That is…until I whipped out my Bissell

PonyGTrider

AeroCoupe

lube between the nut and the face. I know my lubes

PonyGTrider

86HO5.0

Active Member

PonyGTrider

AeroCoupe

lube between the nut and the face. I know my lubes

PonyGTrider

AeroCoupe

lube between the nut and the face. I know my lubes

PonyGTrider

Mustang5L5

That is…until I whipped out my Bissell

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Brakes FOX BRAKES ISSUES

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

lube between the nut and the face. I know my lubes

That is…until I whipped out my Bissell

That is…until I whipped out my Bissell

lube between the nut and the face. I know my lubes

Active Member

lube between the nut and the face. I know my lubes

lube between the nut and the face. I know my lubes

That is…until I whipped out my Bissell

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