Cobra Brake Swap Issues

[97CobraLSC]

I recently bought a Mark VIII that already had the Cobra 13" brake swap on the front.

When I first got the car, it would pull hard left during any moderate braking. I attributed it to a stretched soft line on the passenger side. I replaced the soft lines on both sides and that problem went away.

A problem that it continues to have is no braking power. If you ease into the brakes it stops better than if you nail the brake pedal. It's almost as if the brakes fade instantly.

Any ideas? I don't think the calipers or lines are bad. I am thinking it's the brake booster, but I am not sure.

Help is appreciated! Let me know if you need more info for diagnosis.

97 Mark VIII LSC Cobra PBR calipers 13" front brakes, stock rear.

 

[driller]

Start by bleeding old fluid out each caliper until fresh fluid is coming out. Work from the furthest caliper from the ABS unit in succession to the closest.

Speed bleeders are great for this.

 

[97CobraLSC]

We didn't bleed the rear when we replaced the front lines. We'll try that. I assume it's RR, RL, FR, FL?

This problem happened before I replaced the lines. Does the ABS pump need to run to complete a good bleed?

 

[97CobraLSC]

THIS link goes to the thread on LVC of the previous owner's issues with the brakes.

Hoping I can figure it out. Master cylinder and booster were already replaced. I'm guessing it leaves air in the lines or ballooning rear lines / stuck calipers.

 

[driller]

We didn't bleed the rear when we replaced the front lines. We'll try that. I assume it's RR, RL, FR, FL?

This problem happened before I replaced the lines. Does the ABS pump need to run to complete a good bleed?

Yes, RR,RL, FR and FL.

I've never had any ABS complications when replacing lines or calipers. When I replaced the hard lines on the '93, I crimped the old line shut near the front connection and removed the old line and replaced with the new line. That way I could make the connection to the new line in a minimal amount of time. Then it was simply a matter of bleeding the brakes while keeping the brake reservoir full.

 

[Mad1stGen]

Does your booster keep VAC overnight ?

 

[tonybrooklyn]

I recently bought a Mark VIII that already had the Cobra 13" brake swap on the front.

When I first got the car, it would pull hard left during any moderate braking. I attributed it to a stretched soft line on the passenger side. I replaced the soft lines on both sides and that problem went away.

A problem that it continues to have is no braking power. If you ease into the brakes it stops better than if you nail the brake pedal. It's almost as if the brakes fade instantly.

Any ideas? I don't think the calipers or lines are bad. I am thinking it's the brake booster, but I am not sure.

Help is appreciated! Let me know if you need more info for diagnosis.

97 Mark VIII LSC Cobra PBR calipers 13" front brakes, stock rear.

I had a similar problem on my 97 Continental and 2002 Continental. Both front wheel drives the worst was backing down a steep incline in reverse. Pedal to the floor it would just barely stop. Or hitting the brakes hard at high speed. It was an instantaneous fading on the 97 i changed every single part in the end it turned out to be the per-portioning valve. On the 2002 i had already learned the hard way on the 97 so it was an easy fix. Never had any issues with the Mark, hope this helps rather than confuses! GOOD LUCK!

 

[driller]

...in the end it turned out to be the proportioning valve.

Fixed it for you...

and I agree it could cause such issues once you think about it.

 

[97CobraLSC]

Does your booster keep VAC overnight ?

How would I check this? Pull the vac line running to it and see if it make a sucking sound after it sat?

I assume you're implying vac leak in the hose going to the booster.

Fixed it for you...

and I agree it could cause such issues once you think about it.

Where is it on the Gen 2 and how would I replace it?

 

[Mad1stGen]

How would I check this? Pull the vac line running to it and see if it make a sucking sound after it sat?
I assume you're implying vac leak in the hose going to the booster.

The fact that it was replaced doesn't mean anything, especially if a used part was installed.

Pull the gray 90 deg fitting that's on the booster, to the right of master cylinder. You should hear vac sound as the air fills the booster.

 

[97CobraLSC]

The fact that it was replaced doesn't mean anything, especially if a used part was installed.

Pull the gray 90 deg fitting that's on the booster, to the right of master cylinder. You should hear vac sound as the air fills the booster.

I'll give that a try. Thanks, J.

 

[97CobraLSC]

Another GMAN / gtshaw thread pointing to the same issue that I found. This is still the same car.

http://www.lincolnvscadillac.com/forum/showthread.php?87892-Braking-Issues

 

[97CobraLSC]

Pulled the fitting off right after it ran, no vac sound. Started the car with the vac line disconnected, rough idle as expected. Reconnected, smooth idle like normal, no hissing sounds.

Seems like it's not a leak in the vacuum line itself. I don't know if this means the booster is bad or not.

In the car the pedal is hard at all times. I can push on it as hard as I want to stop and it won't stop any harder.

 

[chris2523]

In the car the pedal is hard at all times.

ok. so brake booster, or vac to brake booster. like i said in february.

 

[driller]

In the car the pedal is hard at all times. I can push on it as hard as I want to stop and it won't stop any harder.

Try this...

With the car OFF, depress the brake pedal moderately a half dozen times. This should get rid of any residual vacuum and the pedal should feel "stiff". Hold a moderate amount of force on the brake pedal and start the car.

You should feel a softer pedal and the same force should cause the pedal to travel further with the engine running supplying vacuum.

While not 100% definitive to rule out the booster, it will be a definite fail for the vacuum booster if the pedal does not soften and travel further with vacuum - assuming no issues with vacuum getting to the brake booster.

 

[97CobraLSC]

ok. so brake booster, or vac to brake booster. like i said in february.

It was sounding like it was the booster, but read my reply below.

Try this...

With the car OFF, depress the brake pedal moderately a half dozen times. This should get rid of any residual vacuum and the pedal should feel "stiff". Hold a moderate amount of force on the brake pedal and start the car.

You should feel a softer pedal and the same force should cause the pedal to travel further with the engine running supplying vacuum.

While not 100% definitive to rule out the booster, it will be a definite fail for the vacuum booster if the pedal does not soften and travel further with vacuum - assuming no issues with vacuum getting to the brake booster.

Did exactly this. Pedal DID soften and travel further once the car was started. So that should rule out the booster, right? I'm not too sure what else is left.

If I didn't mention it before, the car has stock Mark VIII brakes in the rear. I'm not sure if that makes a difference, but I don't think it makes any difference because the front brakes should still be able to hold the car I think.

 

[driller]

Did exactly this. Pedal DID soften and travel further once the car was started. So that should rule out the booster, right? I'm not too sure what else is left.

That does indicate the vacuum booster is working. The only question is if it is working as it should - by that meaning is it providing 'enough' boost.

If I didn't mention it before, the car has stock Mark VIII brakes in the rear. I'm not sure if that makes a difference, but I don't think it makes any difference because the front brakes should still be able to hold the car I think.

I have read many articles in the past concerning brakes and the technical background when modifying brake systems. The biggest factor I came away with is there is no one magic combination and it varies from car to car based on many factors such as overall weight, weight balance, wheel size, etc... not to mention the myriad of choices on calipers, pads and rotors.

That being said, many Mark VIII owners (including myself) have the front Cobra brake upgrades with stock rear calipers and possibly stock rear rotors. As a general rule, most seem to be completely satisfied with the upgrade in performance. The front brakes on my '93 will hold the car up to 2500 rpms or so at the track with my current setup. There is no reason to expect any less on yours, so that leads me to believe there is definitely something amiss.

I'm not sure where the brake proportioning valve on a Gen2 is but I recall DLF posting pics of his and I think it was mounted somewhere in the rear? I know when I replaced the hard lines on my '93 I did not find such a valve.

Since the proportioning valve is supposed to reduce pressure to the rear brakes, it sounds logical if it was malfunctioning there would be more braking pressure to the rear brakes and by default less brake pressure to the fronts where it is needed most? (I only put a question mark there since it is purely my supposition and not necessarily fact)

 

[97CobraLSC]

That does indicate the vacuum booster is working. The only question is if it is working as it should - by that meaning is it providing 'enough' boost.



I have read many articles in the past concerning brakes and the technical background when modifying brake systems. The biggest factor I came away with is there is no one magic combination and it varies from car to car based on many factors such as overall weight, weight balance, wheel size, etc... not to mention the myriad of choices on calipers, pads and rotors.

That being said, many Mark VIII owners (including myself) have the front Cobra brake upgrades with stock rear calipers and possibly stock rear rotors. As a general rule, most seem to be completely satisfied with the upgrade in performance. The front brakes on my '93 will hold the car up to 2500 rpms or so at the track with my current setup. There is no reason to expect any less on yours, so that leads me to believe there is definitely something amiss.

I'm not sure where the brake proportioning valve on a Gen2 is but I recall DLF posting pics of his and I think it was mounted somewhere in the rear? I know when I replaced the hard lines on my '93 I did not find such a valve.

Since the proportioning valve is supposed to reduce pressure to the rear brakes, it sounds logical if it was malfunctioning there would be more braking pressure to the rear brakes and by default less brake pressure to the fronts where it is needed most? (I only put a question mark there since it is purely my supposition and not necessarily fact)

I was thinking just that. I thought that I saw the proportioning valve in the driver's front fenderwell, but I'll check Ford PTS. I know that the valve is discontinued, so finding a new one (NOS?) might be hard. Probably a junkyard find.

I know the car should stop harder than it is. I am not great at automotive diagnostics, so this is beyond my brain. (I'm an IT guy).

 

[97CobraLSC]

Just thought of something...

J said that I should've heard a vacuum sound when I pulled the line with the car off. I never heard that. According to PTS, there should be a check valve that keeps vacuum when there is reduced vacuum. Could that be the culprit?

Just throwing things out there...

 

[Ford nut]

ok. so brake booster, or vac to brake booster. like i said in february.

G-man took one off a parts car according to the thread he posted.
Who knows if it was installed correctly, or if it was any good.

The car had the issue before the brake upgrade, and was never fixed.
Its not the cobra brakes, get good booster have it installed correctly.
Sorry but it looks like he just sold you his problem.
Got to love it.....