Fox Restricted Brakes


Active Member
Nov 26, 2020
My car is a 1993 with the factory 4 cylinder brakes. It is a convertible, so it uses the V8 booster, even though the rest of the system is 4 cylinder.

My symptoms... This is hard to explain, bear with me:
My brake pedal cannot move quickly. Even if I press hard on the pedal, it will take about 2 seconds to reach its desired position. Its almost like there is a restriction in the system preventing fluid from moving quickly. I am 99% sure I do not have air in the system. I have bled it over and over again.
In a normal vehicle, if you hammer the brakes, you get an instant response. But I have to hold constant pressure and wait while the pedal travels.

Here is what I've done:
  • New soft lines
  • New brake drum cylinders
  • New brake pads and brake shoes
  • Resurfaced rotors
  • New master cylinder (bench bled)
  • New brake booster vacuum line (full manifold vacuum)
  • They've been bled many times.
I have a B303 cam, but I still get plenty of vac signal. The brakes do not pull to either side.
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Since the diver's side caliper does not go through the combination valve, wouldn't that make the car pull to the left if a restriction was present in the valve?

Also, when I press the brake, there is a hiss from the brake booster. It isn't a sustained hiss, but just a singular "pshhh" noise at the pedal moves.
It’s possible the restriction is in the rear line, which wouldn’t affect the two fronts.

Not saying this is 100% the cause, but as you check other things one thing to keep in mind.

How do the brakes feel with the engine off? How’s pedal and travel distance?
I tore the proportioning valve apart and cleaned it. No affect.

Should I be able to blow air through both inputs of the valve? I can.

Now, the brake booster rod. My brakes don’t drag, nor is there any pedal travel before the brakes begin to apply.

Things are pointing towards the booster itself. I have 13 inches of idle vacuum.
15-18 inches is typical for a vacuum system to work well. 13 is close.

do you have the cam installed straight up? Sometimes timing can play a role in vacuum level.

Do the brakes feel any different at higher rpm?
Brake booster binding? Pedal bind? Does it feel like a physical restriction on the pedal?

What parts have not been replaced since you have been trying to correct this issue?

But you said you had the issue and replaced the booster? But still the problem remains?

Sorry to spitball, but this is kinda one of those things I wish I could inspect in person.
No binding. Pedal feels good. I’ve never replaced the booster. I just ensured it was getting full manifold vacuum and the check valve was working. Booster is original as far as I know.

The only things not replaced are the calipers, proportioning valve, booster, hard lines.
Sounds like the booster is bad. Pedal should get immediately easier to push when the motor is running due to the booster aiding in the required foot force for braking. The only think that seems to be changing is with the motor running the pedal no longer moves downward smoothly / takes more time. This to me indicates that booster is done.

If you ever see yourself going to the SN95 brakes and you determine it is the booster you may want to look at installing the 1993 Cobra booster.
For some reason I i read the initial post and thought that it said you did replace the booster. But now i see it was just the line.

At this point i would suspect the booster as well, but I hate firing the parts cannon off without confirming. Only other suggestion would be to maybe check the booster check valve for proper operation. The grommets can leak, or the check valve can fail to prevent flow to the vac source. You can pull that out and test by just blowing through it.
The booster is definitely doing something. It gets easier with engine running. I just don’t think it’s doing enough.

Almost like it’s slow to assist… That’s why I thought there might be a fluid restriction.

I tried a full force stop last night on a soaking wet road and couldn’t lock any tires up.
When you bled the brakes. How was fluid flow out of the bleeders?

You said you replaced all the soft lines The 2 fronts and the axle line? SO that would rule out collapsed hose.