Problems with Cobra brake swap

Topic: Problems with Cobra brake swap (Read 4571 times) previous topic - next topic

Problems with Cobra brake swap

February 04, 2021, 10:44:07 PM

Swapped Cobra brakes on a while ago and it's been absolute hell trying to get them right. I have later cobra front brakes, sn95 gt rear brakes, 94 Cobra master and a fox booster. Now no one has been able to figure out why I have the worlds softest and worst pedal feel. half the pedal is dead, but I can still lock up my brakes on the highway. I have checked booster rod clearance and adjusted it. the brakes have been bled in every way including pressure bleed. I have put a total of 6 gallons of brake fluid through this system trying to figure it out. 4 mechanics have tried to figure it out. The only bandaid that we have thought up was to just adjust out the booster pushrod to where it preloaded the master. we pressure bled them with the new machine at work and it just absolutely made them even worse than they were recently. any ideas, any help would be very appreciated.

Re: Problems with Cobra brake swap

Reply #1 – February 05, 2021, 03:10:49 AM

What sort of proportioning valve are you using? Does the 94 cobra master require changing the pedal ratio?

Re: Problems with Cobra brake swap

Reply #2 – February 05, 2021, 09:41:57 AM

X2 on prop valve...

Re: Problems with Cobra brake swap

Reply #3 – February 05, 2021, 09:43:46 AM

I did a quick google search and the master cylinder size is 15/16" Isn't that too small for the Cobra brakes and Sn95 rears?

Also what bodyman asked about the pedal ratio?

Re: Problems with Cobra brake swap

Reply #4 – February 05, 2021, 07:44:22 PM

Quote

I did a quick google search and the master cylinder size is 15/16" Isn't that too small for the Cobra brakes and Sn95 rears?

Looks like the 99-04 masters are 1 1/16” bore. Just changing the master to the 99-04 may help pedal feel some. Here is what Jack Hidley (maximum motor sports) had to say about pedal ratios on my thread.

Quote

I think you are going to have one problem with the hydroboost installation. All of the 1987-88 T-Birds have a 3.5:1 pedal ratio. That is the pedal pad moves 3.5 units for each unit that it pushes the m/c pushrod. I do not have the data for your 1985 2.3l turbo, but I assume it is the same. You can check you pedal ratio be measuring the physical brake pedal dimensions and dividing one by the other.

The Mustang that you pulled the hydroboost unit from has a pedal ratio of 4.75:1.

Since you appear to have connected the hydroboost unit pushrod to the 3.5:1 pedal pin, the required pedal effort and travel is going to be quite a bit different than what Ford designed. The brake pedal effort is going to be 36% (4.75/3.5) higher. The travel at the pedal pad is going to be reduced by the same amount.

To correctly fix this, the pin on the brake pedal should be moved upwards slightly less than 1" (0.97") and the hydroboost unit should be raised on the firewall to correctly align the pushrod so it is straight.

Re: Problems with Cobra brake swap

Reply #5 – February 05, 2021, 09:50:13 PM

Wilwood proportioning valve. The sn95 gt rear brakes are the same the only difference is the caliper bracket is connected and the pads are thinner to compensate for the vented rotor. 15/16 is considered by many to be ideal for the cobra brake swap. The pedal feel is extremely soft and a lot of dead pedal. According to what everyone is saying on other forums I should have a decently stiff pedal. the lead tech at my shop did all the calculations for master sizing and said that its the right size for the calipers. its stumped literally everyone. I was talking to the shop manager and he was saying I may have gotten multiple faulty master cylinders as it took 3 to get his brakes to even work. All of these units have been from oriellys. I had him order a stock 88 thunderbird master from autozone. This will make the 5th total master this car has seen. Now what I have been reading is that the larger the master cylinder bore the greater the pedal effort. So in theory with a fox mustang booster and a 88 bird master it should be absolutely miserable. Lets hope so.

Re: Problems with Cobra brake swap

Reply #6 – February 06, 2021, 02:51:09 PM

The proportioning valve was put in place of the original or are you running the original and the Wilwood?

Re: Problems with Cobra brake swap

Reply #7 – February 06, 2021, 11:00:37 PM

original proportioning valve has been deleted just the wilwood

Re: Problems with Cobra brake swap

Reply #8 – February 07, 2021, 03:41:19 AM

I also got three bad brake master cylinders in a row from parts stores. Two reman and one new all in the same go.

The one that was on the car felt find but bled off when you held the pedal down. If you stepped on it again it would do the same thing.

All three of them I couldn't get a stiff pedal out of and the fourth one worked perfectly with almost no bleeding.

Re: Problems with Cobra brake swap

Reply #9 – February 08, 2021, 08:14:08 PM

If its not OE or high dollar aftermarket i'm finding all replacement parts are absolute garbage. The parts stores are selling the absolute worst parts. It's getting bad. I had to do 3 starters in a Cherokee at work because the starters were going out in about a weeks time. Finally we got one that was worth a .

Re: Problems with Cobra brake swap

Reply #10 – February 09, 2021, 03:44:24 PM

So here is how both my 93 Coupe was plumbed for the Cobra brakes. The first round of brakes was a full blown 95 GT brake swap including the ABS. I pulled everything off of the 95GT that I would need for the swap so the MC & booster cam off the firewall. Off the front of the car I took the front spindles with hubs, dust shields, rotors and calipers. Of the back of the car I took everything from the axles out so axles, calipers, rotors, caliper brackets, axle tube brackets, and dust shields.

I used the '95 brake booster out of the car and got a reman MC for a 95GT which has a 15/16 bore. I gutted the stock Fox combination valve (proportioning / shuttle) on the proportioning side and installed a Wilwood manual proportioning valve to the rear brakes. Screwing the adjusting all the way in (clockwise) produces zero reduction in pressure. I then backed it out four full turns based on previous experience with other Foxbody Mustangs. Installed the SN95 spindles, dust shields, new hubs, turned rotors, remaned '99 GT twin piston calipers, new pads, and the flexible lines to the front calipers were swapped to Maximum Motorsport SS braided lines.

The body hard line to rear end line and the hard lines to rear calipers were also swapped out to SS braided lines from Maximum Motorsports. I pulled the stock Fox axles and all the Fox drum brake stuff and into the metal s bin they went along with all of the stock Fox front brake parts. Installed the SN95 axle brackets, dust shields, axles, caliper brackets, turned rotors, calipers, new pads and connected the MM SS braided lines.

Adjusted the booster brake rod so that it was almost touching the MC plunger and I mean almost. I have a brake booster rod gauge that I use and suggest you buy one.

https://www.ecklers.com/power-brake-booster-tool-71-397920-1.html?utm_source=google&utm_medium=cpc&adpos=&scid=scplp71-397920-1&sc_intid=71-397920-1&gclid=Cj0KCQiAgomBBhDXARIsAFNyUqOFDvig91cPbd3fNi8cMlEVQ2qIOHEoTcztTQkTxJUNwRbkx8BDzksaAosyEALw_wcB

Then it was a lot of work plumbing in the ABS and wiring the sensors so we will skip that but it does work and anyone that says they can manually modulate the brakes better is full of .

Next I bled the system with a power bleeder. Again, get one and just be done with it. They are mandatory on ABS systems and should be on non-ABS systems as gone are the days of pumping the brake pedal.

https://www.motiveproducts.com/collections/domestic-bleeder-kits

Dropped it on the ground and about choked myself with the seat belt the first time I hit the brakes.

Note - My Coupe was a 4 cylinder car converted to a V-8 car. I used an 8.8 out of an '87 GT Mustang and then sourced the body hard line and bracket for the body hard line to rear end soft line off of a salvage yard car.

Re: Problems with Cobra brake swap

Reply #11 – February 09, 2021, 04:25:20 PM

So with all of that anytime I get a mushy brake pedal it is one of two things if there are no leaks. MC or booster and I would say its most likely the MC but I have had two bad boosters.

The use of the Fox booster with the '94 Cobra MC is not a good choice. The Fox booster cannot deliver enough boost to compensate for the additional pressure it takes to move the 15/16" bore '94 Cobra MC. The 93 Cobra MC is a 1" bore, the '95 GT MC is 1-1/16" bore, and a '93 GT MC is 21mm (between 13/16" and 27/64"). As far as I know the 83-88 Birds and Cougars (other than the 87-88 turbo cars) came with a 21mm MC as well. So with all of that you can see that a stock fox booster was meant to be coupled to a 21mm bore MC. The 15/16" bore MC is roughtly 13.4% larger in area, the 1" bore MC is roughly 21% larger in area, and the 1-1/16" bore MC is roughly 46.3% larger in area than the stock 21mm unit. So with that I would suggest not mixing and matching but either run a Cobra MC with a Cobra booster or SN95 GT/V6 Booster with SN95 GT/V6 MC. The 93 Cobra MC will work just fine with the SN95 GT/V6 booster as many have gone this route so they did not have to change the brake line fittings being the 93' Cobra MC is SAE threads and the SN95 GT/V6 MC is metric.

So with the larger area on the MC bores you can see that they will move more fluid which the larger front Cobra calipers require. This is also true if you are going from rear drum brakes to calipers. And the larger area also equals more pedal pressure because you are not changing the pedal ratio hence the need for more boost.

Note - The reason the '93 Cobra runs a 1" MC is the calipers are not the same as the 94+ Cobras. I know the rears are the Turbo Coupe calipers and the fronts are the same as the GT cars with the exception of the rotors having a taller hat which I believe was done to use an 8.5" wide wheel. The 93 Cobra R uses the same exact brakes as the 94-95 Cobra.

If you use an SN95 booster you will have to slot the holes on the firewall or get a reman 93 Cobra booster as the mounting studs on it were meant to fit the existing firewall holes. Massaging the driver side strut tower may be required with the 93 Cobra Booster (seems to be hit and miss) but the SN95 booster will most definitely require it and it can be a bitch with the motor in the car.

So back to your problem. How did you run the brake lines from your MC to the brake calipers if you no longer have the stock combo valve? I will wait to go any further until I understand what you did here.

Re: Problems with Cobra brake swap

Reply #12 – February 09, 2021, 09:03:35 PM

Alright, so when I read the bore diameter when looking at master cylinders. It seems they messed up the listing because it said that it was 1.183 bore. Now looking at other ones it is definitely not. So I will pick up another master.

Now I do not understand how a bad brake booster would cause a mushy soft pedal. Can you please inform me on this

The system has been manually flushed, then I used a power flusher after I bought one. It was then power flushed with a machine that we have at work. The pushrod length was checked with a digital caliper and then I double checked it with a booster pushrod length tool later on.

The stock proportioning valve has been removed. The two front lines are connected via a T to the master and the rear is ran with just a Wilwood manual proportioning valve.

Re: Problems with Cobra brake swap

Reply #13 – February 11, 2021, 06:21:19 AM

You want the SN95 V6 booster, and the 93 cobra master. Worked for me.

Re: Problems with Cobra brake swap

Reply #14 – February 16, 2021, 11:50:14 AM

So there are a couple things you need to check of which revolves around getting the proper vacuum to the booster. The valve on your booster where the vacuum line plugs in is a one-way valve so I would start there. See if the seal is in good shape. I can pop mine out and you can check the one way action by blowing into the vacuum line connection side and then blowing into the booster side. If it checks out then inspect the seal and if that is good pop it back in. Next with the engine off, pump the brakes several times to remove any residual vacuum in the booster. Now push the brake pedal firmly down and hold pressure on the pedal and start the engine. You should feel the pedal go down slightly if and when adequate vacuum reaches the booster. I have always gone off of if the pedal doesn't move at all, comes back at me, or if it goes down over an inch or more there is probably an issue. I have seen them fail due to brake fluid leaking out of the MC and getting into the booster causing issues.

If you have one install a vacuum gauge in the line at the booster, start the engine and check the available vacuum. Typically you need around 17-22” of vacuum for the booster (and everything else in the car) to work right. I think you can borrow a gauge from the parts stores if need be.

The other thing I have seen is plumbing the MC incorrectly. Typically the primary port is the one closest to the booster and should be plumbed to the front brakes. This is due to the primary moving more fluid than the secondary which the larger front brakes need. With a proper functioning MC this could cause the front brakes to not receive the volume or pressure necessary to work 100%.

The only way I know a booster can cause a mushy pedal is for the booster rod not being properly adjusted but it sounds like you have that covered. Booster is just a multiplier for manual brakes. To get the power you would need from a manual brake setup you would have to change the pedal ratio i.e. lever arm principle.