Installed Cobra brakes, still can't stop. Going hydroboost

[1slow95]

Bullitt brakes actually, but they're the same thing. Bullitt calipers, Hawk HPS pads, stock replacement 13" rotors, and SS brake lines. It does stop better, but it's not nearly as good as I was expecting. I know a big part of the problem is that I'm only pulling 11-12 in.hg. vacuum at idle. So I just bought a complete hydroboost setup from a 99-04.

Other than the lack of vacuum assist, what could cause poor braking? I bled the system and pretty sure I got all the old fluid flushed out. The pedal is very stiff. I replaced the booster when I had the engine out a few years ago, but the master cylinder is the stock 172k mile piece. The hydroboost setup will replace the mc so if that's the problem then I'm good to go.

The problem is when I mash the brakes, they don't feel like they're grabbing. It doesn't even attempt to lock up the tires(ABS doesn't work BTW, rear speed sensors broke). The brakes in my Jeep feel stronger, even with the new Cobra brakes. The pedal will only go about halfway before it stops, which is why I'm thinking it's a power assist problem.

Here's a couple pics from the swap.

 

[revhead347]

I had similiar problems. I think you're dead on with the vacuum. I was only pulling 9.5" on mine. You really need about 16" of vacuum for the brake booster to work properly. I ended up going manual brakes. It was a bad idea, and only makes it mildly easier to stop. I think you are on the right track with the hydroboost.

Kurt

 

[wytstang]

but the master cylinder is the stock 172k mile piece.

Their is your problem, you need the cobra mc (15/16 bore vs. the gt 1 1/16 bore) for the cobra brakes to work properly. That's exactly the same reason why you are experiencing the stiff brake pedal. Buy these adapter line from MM Master Cylinder Adapter, 94-95 Cobra master cylinder, 94-95 with Stock Proportining Valve [MMBAK-7] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader! and then the correct MC and your braking problems are done.

 

[1slow95]

Their is your problem, you need the cobra mc (15/16 bore vs. the gt 1 1/16 bore) for the cobra brakes to work properly. That's exactly the same reason why you are experiencing the stiff brake pedal. Buy these adapter line from MM Master Cylinder Adapter, 94-95 Cobra master cylinder, 94-95 with Stock Proportining Valve [MMBAK-7] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader! and then the correct MC and your braking problems are done.

I know the stock MC is why the pedal is hard, but wouldn't the larger bore MC equal more pressure? Basically, the stock MC has the same braking power at half pedal as the Cobra MC at 3/4 pedal?

The hydroboost setup comes with a MC from a 99-04 GT, which is the same as the Cobra MC.

 

[sixt9coug]

Smaller bore will equal more pressure

 

[nmcgrawj]

Unfortunately there is a lot of rumors out there about GT's upgrading to Cobra front brakes. I had a VERY long email string with a tech at MM....


putting Cobra front brakes on a GT with stock rear brakes makes the car perform WORSE than the GT brakes itself.



This coming from the place we all respect for their parts and service. If i would have known that, i would have just waited and bought an aftermarket set up.

 

[Adam95GT]

Unfortunately there is a lot of rumors out there about GT's upgrading to Cobra front brakes. I had a VERY long email string with a tech at MM....


putting Cobra front brakes on a GT with stock rear brakes makes the car perform WORSE than the GT brakes itself.



This coming from the place we all respect for their parts and service. If i would have known that, i would have just waited and bought an aftermarket set up.

Exactly why i kept my stock brakes...

 

[Bullitt95]

Unfortunately there is a lot of rumors out there about GT's upgrading to Cobra front brakes. I had a VERY long email string with a tech at MM....

putting Cobra front brakes on a GT with stock rear brakes makes the car perform WORSE than the GT brakes itself.

I swapped the front brakes from a '98 Cobra onto my GT (kept the stock MC) and they perform much BETTER than the stock GT front brakes. Night and day difference!

 

[revhead347]

Stock brakes are aweful.

Kurt

 

[1slow95]

Unfortunately there is a lot of rumors out there about GT's upgrading to Cobra front brakes. I had a VERY long email string with a tech at MM....


putting Cobra front brakes on a GT with stock rear brakes makes the car perform WORSE than the GT brakes itself.



This coming from the place we all respect for their parts and service. If i would have known that, i would have just waited and bought an aftermarket set up.

I thought the rear calipers were the same between the GT and Cobra, and the Cobra just has a larger vented disc?

I never said my brakes were worse than stock. They're definitely better, just not as good as I was expecting. My stock brakes were almost worthless, they actually scared me a couple times.

 

[Mustang5L5]

I know the stock MC is why the pedal is hard, but wouldn't the larger bore MC equal more pressure? Basically, the stock MC has the same braking power at half pedal as the Cobra MC at 3/4 pedal?

No, that's not how it works. Bigger is not always better as in this case.

Basically, part of the cobra brake conversion on a 94-95 is that you NEED a new master cylinder. I'm surprised at just how many 94-95 guys don't realize this. Most guys complaining that the Cobra brakes weren't that great on their GT's after the swap never changed the MC out.

Even Ford engineers recognize this which is why the M-2300-Q kit was sold. It's Cobra 13" brakes with the new required master cylinder

Ford Racing M-2300-Q Ford Racing Mustang Brake Upgrade Kits


Reason you need a smaller bore MC here is because, beleive it or not, the cobra calipers are SMALLER than the stock 66 units you replaced.

Here some numbers

2x 66mm calipers = 6842mm^2 of piston area

2x 99-04 Cobra/Mach/Bullitt 40.5mm calipers = 5152mm^2 of piston area.

2x 94-98 Cobra calipers 38mm = 4536mm^2

In terms of hydraulics, that's a HUGE differnce. You are decreasing the slave cylinder (caliper) piston area and making the master and slave closer in ratio. That requires MORE force from your leg to achieve the same braking pressure. In order to compensate mechanically, you need to reduce the size of the MC's bore in order to keep the same appropriate hydraulic ratio.


This is where things get tricky. The 94-95 Cobra 15/16" MC was developed for the 94-98 Cobra calipers which feature 38mm pistons and an even smaller surface area of 4536mm^2. The 99+ casting of the 13" Cobra/Bullitt/Mach caliper has more surface area, so use of the 15/16" MC may result in a soft pedal with a long travel.

The 94-95 GT MC is 1 1/16".

I basically have the same setup as you do on my Fox Mustang. Mach 1 front calipers, Mach 1 rears. I ran the 94-95 GT MC (because it's what i had at the time) and the brakes SUCKED. I swapped to a smaller 1993 Cobra MC 1" bore (no brake lines need to be changed...direct plug in) and the braking was MUCH better. The 15/16" MC is ideal for the 94-98 Calipers, but i went with the 1" bore because the 99-04 versions have more piston surface area. I like the pedal feel with the 1", some guys on the fox boards like that setup with the 15/16" MC instead.

To swap in a 94-95 Cobra 15/16" MC, you will need a $20 brake line adapter kit from maximum motorsports. MMBAK-7

Master Cylinder Adapter, 94-95 Cobra master cylinder, 94-95 with Stock Proportining Valve [MMBAK-7] : Maximum Motorsports, the Latemodel Mustang Performance Suspension Leader!

The 1993 Cobra 1" MC is a direct swap in with no line changes needed.

I'm not telling you which MC you should go with, but just stating that the MC NEEDS to be changed when installing 13" brakes on a 94-95 GT. Without it, you won't be able to put as much hydraulic force into the calipers and then what's the point of the upgrade?

Hydroboost does offer way more assist than vacuum brakes, but you'll need to swap the booster, MC, run new lines and run the power steering hoses, etc. The gt master cylinder is 1 1/16, but the cobra and mach use the same size in hydroboost. A master cylinder swap is MUCH easier. Yoir vacuum numbers are idle are not that critical as overall vacuum at cruise. If you can pull good vacuum during cruise, it should build up even stored vacuum for several stops and your brakes should work fine.

 

[revhead347]

I've actually heard of, and it's been discussed here before, of people putting Cobra brakes on with their stock master cylinder, and it worked ok. The Cobra rear rotors are bigger, but the caliper is the same.

Kurt

 

[95BlueStallion]

I have Cobra front brakes with stock rears, and it is MUCH better than stock. I kept the stock GT MC also. I like the firm pedal, and have no complaints for $ spent.

 

[nmcgrawj]

I have Cobra front brakes with stock rears, and it is MUCH better than stock. I kept the stock GT MC also. I like the firm pedal, and have no complaints for $ spent.

So how much shorter is the stopping distance?



Or are we making statements by the "butt meter".

 

[MFE92]

Look at that last pic. Your pads are barely contacting the rotor enough to wear away part of the surface rust.

 

[nmcgrawj]

Using Cobra front brakes with GT rear brakes will increase your stopping distance. In other words, a Mustang equipped with this brake combination will not stop as well as it did from the factory. This is a well-documented and scientifically measured fact.

A large percentage of the cars in question have factory ABS. This deals with the mismatched brake bias fairly well, but not perfectly. If the car doesn't have ABS on it, having the GT brakes on the rear is going to increase the front brake bias about 10%. This will increase stopping distances about 5%. When someone installs the Cobra front brakes on their GT, the car will require less pedal effort for a given deceleration rate. Most people will feel this and say that the car stops "better". However, when actually measure to find out what the minimum stopping distance the car is capable of, it will be about 5% longer. Less brake pedal effort does not mean better braking. Minimum braking distances can only be achieved by having the correct front/rear brake bias. See below for more details.

Jason Jacques
805.544.8748 x 13
Maximum Motorsports, Inc.
street | drag | auto-x | road race

I think he was referring to the fact that if you slam on the brakes as hard as possible, the wheels will want to lock up causing the stopping distance to be greater. ABS will fight this, thus making it not as bad. But still not making up for it.

I also sent jason another email asking a few more questions, will report back with details.

 

[1slow95]

I'm not telling you which MC you should go with, but just stating that the MC NEEDS to be changed when installing 13" brakes on a 94-95 GT. Without it, you won't be able to put as much hydraulic force into the calipers and then what's the point of the upgrade?

Hydroboost does offer way more assist than vacuum brakes, but you'll need to swap the booster, MC, run new lines and run the power steering hoses, etc. The gt master cylinder is 1 1/16, but the cobra and mach use the same size in hydroboost. A master cylinder swap is MUCH easier. Yoir vacuum numbers are idle are not that critical as overall vacuum at cruise. If you can pull good vacuum during cruise, it should build up even stored vacuum for several stops and your brakes should work fine.

Great info. In my searches before buying the brakes I found many people reported the stock MC and Cobra brakes worked fine, and many others that said it was much better with the Cobra MC. I figured I'd try it and buy the MC if I needed it.

I just checked and the hydroboost setup I bought is off an '04 Mach 1 so I'm good to go on the MC. On the vacuum problem, stopping at idle I get one good pump of the brake pedal then power assist starts going away.


MFE, the surface rust is there because I had just washed and waxed the car.

 

[nmcgrawj]

Do a detailed install of the hydroboost will ya? Take lots of pics!

 

[Mustang5L5]

. On the vacuum problem, stopping at idle I get one good pump of the brake pedal then power assist starts going away.

Have you considered manual brakes? You'd need a specially designed kit, like from maximum motorsports which includes a revised brake pedal to alter the pedal ratio. You would need an MC change as well, but much more simplier than hydroboost.

Your choice on how you want to go. I see three options

Swap MC and retain the vacuum booster
swap MC and swap to manual brakes
plumb in the hydroboost.

Your call.

 

[revhead347]

I got manual brakes. You actually just drill another hole in the stock brake pedal higher up. I don't recommend it. It really makes the car hard to stop.

Kurt