Fox Brakes sticking after booster rep.

Maciasmarci

Member
May 19, 2021
46
1
8
Stockton
Hi, I have a 1990 5.0 with 99-04 mustang spindles/ caliper conversion. I am recently experiencing brake sticking at the front when I drive for about 15-20 mins. I’ve recently replaced the stock brake booster for a 1993 cobra booster and thought it was the adjusting rod but I backed it all the way off to eliminate that being the issue. After that I replaced my brake calipers ,rotors, pads and both cables… bled air and took for a test drive. After 15-20 mins I had the same brake smell and car was still braking on its own as I experienced prior to replacing parts. Master cylinder is new . After the drive I jacked the front end and I needed a bit of effort to turn the wheels by hand. Any help much appreciated.!
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Massachusetts
The 99-04 GT/V6 calipers are notorious for pistons sticking, especially brand new remans. It's usually an issue with cars that sit with old brake fluid laiden with moisture.

I don't know the source of your calipers, but if they are used this is most likely the issue. Remove the calipers and try to push the pistons in and not how difficult they might be. Shouldn't take much effort and all four should be equal in force.
 
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AeroCoupe

Founding Member
Oct 28, 2001
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Jenks, OK
When you say you replaced "brake calipers ,rotors, pads and both cables" what do you mean by cables? Are you referring to the rubber line between the caliper and the hard line on the body? Reason I ask is it sounds like collapsed soft lines to me without knowing if all of the components you replaced were new, reman, or just some parts you had lying around as Mustang5L5 is indicating above.
 
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Maciasmarci

Member
May 19, 2021
46
1
8
Stockton
The 99-04 GT/V6 calipers are notorious for pistons sticking, especially brand new remans. It's usually an issue with cars that sit with old brake fluid laiden with moisture.

I don't know the source of your calipers, but if they are used this is most likely the issue. Remove the calipers and try to push the pistons in and not how difficult they might be. Shouldn't take much effort and all four should be equal in force.
The 99-04 GT/V6 calipers are notorious for pistons sticking, especially brand new remans. It's usually an issue with cars that sit with old brake fluid laiden with moisture.

I don't know the source of your calipers, but if they are used this is most likely the issue. Remove the calipers and try to push the pistons in and not how difficult they might be. Shouldn't take much effort and all four should be equal in force.
That may be true . The only reason why I don’t think it’s the calipers is because they can be pushed in with a clamp fairly easy and also the brakes only started doing that after I replaced my brake booster from autozone.
 

Maciasmarci

Member
May 19, 2021
46
1
8
Stockton
When you say you replaced "brake calipers ,rotors, pads and both cables" what do you mean by cables? Are you referring to the rubber line between the caliper and the hard line on the body? Reason I ask is it sounds like collapsed soft lines to me without knowing if all of the components you replaced were new, reman, or just some parts you had lying around as Mustang5L5 is indicating above.
Sorry yes I meant to say I replaced the rubber lines
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Take it for a drive and get it to the point the wheels are locking up. Loosen up the two nuts holding the MC to the booster and slide the MC forward slightly. Do the wheels then free up?

I know you said you backed the rod way off, but this will double check that the booster rod itself isn't somehow sticking and pushing on the MC. Since the issue happened when you changed booster, i would focus on the booster and how it interfaces with the MC. Pedal bind? Booster rod hanging up? That sort of stuff
 
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Maciasmarci

Member
May 19, 2021
46
1
8
Stockton
Take it for a drive and get it to the point the wheels are locking up. Loosen up the two nuts holding the MC to the booster and slide the MC forward slightly. Do the wheels then free up?

I know you said you backed the rod way off, but this will double check that the booster rod itself isn't somehow sticking and pushing on the MC. Since the issue happened when you changed booster, i would focus on the booster and how it interfaces with the MC. Pedal bind? Booster rod hanging up? That sort of stuff
I haven’t tried that but I will today. Thanks
 

AeroCoupe

Founding Member
Oct 28, 2001
547
235
73
Jenks, OK
So its a slow day and this problem caught my curiosity. I am going to throw this out there but I have never seen this so I cannot vouch for it. The combination valve may be failing in such a way that it it holding pressure on the front brakes. Found a couple of instances on a couple sites were the stock one was replaced and it solved the problem. So you understand what I am talking about when I say combination valve it is the brass block that has the proportioning valve and shuttle valve in it and also has a switch in it that illuminates the brake light in the dash if you have a failure.

Fox_Mustang_Combination_Valve.jpg


Most know all of this below is when swapping over to SN95 brakes on a Fox and most know this but just in case.

Typically when installing the SN95 brakes on a Fox the factory proportioning valve side of the combination valve is gutted and a solid plug is installed.

BrakeProportioningValveDiagram.jpg


This is one place to get the plug:


After gutting the proportioning valve side of the combo valve a manual proportioning valve for the rear brakes is installed at the union that is on the firewall or at least that is a super convenient place to put it.

Factory Union
Fox_Mustang_Rear_Brake_Line_Union_on_Firewall.jpg


Aftermarket Manual Proportioning Valve Installed
_Brake_Line_Manual_Proportioning_Valve_on_Firewall.jpg


You can get the manual proportioning valve from several different places but again MM has them. You will need the SAE one as the factory union has 3/8-24 threads. The other thing you will need is a 3-2 conversion kit so you can tie the three lines from your old Fox 3 port master cylinder to the 1993 Cobra or SN95 2 port master cylinder.

You can eliminate the combination valve all together. This will require the 3-2 kit, a manual proportioning valve, and a combination valve elimiantor kit. You install the 3-2 kit with a 1993 Cobra MC or SN95 MC, install the eliminator kit, install the manual proportioning valve for the rear brake line, and then properly bled the system.

All of the Fox to SN95 parts can be found here:


Or you can do what this guy did:

 

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CAMTWO1070

Active Member
Dec 17, 2021
163
48
38
NY
Hi, I have a 1990 5.0 with 99-04 mustang spindles/ caliper conversion. I am recently experiencing brake sticking at the front when I drive for about 15-20 mins. I’ve recently replaced the stock brake booster for a 1993 cobra booster and thought it was the adjusting rod but I backed it all the way off to eliminate that being the issue. After that I replaced my brake calipers ,rotors, pads and both cables… bled air and took for a test drive. After 15-20 mins I had the same brake smell and car was still braking on its own as I experienced prior to replacing parts. Master cylinder is new . After the drive I jacked the front end and I needed a bit of effort to turn the wheels by hand. Any help much appreciated.!
Sounds like you need to replace the rubber hoses going to the front calipers and the rearend....

When they collapse internally they trap the fluid pressure in the calipers and slowly let it bleed off plus as the brakes get hotter they activate more when the brake fluid starts to expand from boiling....

Everytime I replace my brakes in all my vehicles I replace both front rotors plus the rubber hoses and all springs ,clips pins and sliders too....

20160107_113602.jpg



I also replace or repack the wheel bearings plus the wheel bearing races and change the wheel bearing seals plus set wheel bearing lash and replace the brake fluid too.............

Every 100k I go the extra mile and replace both calipers, the brake booster and both rear wheel cylinders along with rear brake shoe backer plates if heavily worn or warped and I also replace the rearend seals and rear axle bearings plus change pinion seal,reset pinion lash and replace the rearend fluid and rear brake drums..

I can honestly say if any of my vehicles is involved in an accident one of the things that cant be questioned is the mechanical ability of my vehicles due to age also normal wear and tear as i replace anything that looks questionable....

Good Luck
 

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I’ve found the issue . As soon as the car gets warm and I drive around the brakes start to stick, so just to make sure it wasn’t a brake pedal issue, I pushed up against it and it seemed to release the brakes and the car would roll back like normal. Thanks a lot for the help.!