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Noob here - I have progress threads on my 1988 GT in a few other forums and have asked this question in those threads but I'm just going to jump to this here, since it's my current predicament.

I have a 1988 GT that I've been working on. I bought it back in January from a guy who was the second owner. I believe most of the mods were done by the first owner. It's been sort of a mixed bag. Some of the mods were done in what might be described as custom, or perhaps a half-assed way. The alternator for example, was upgraded to 3G, but the power cable that runs from the alternator to the starter solenoid, was basically a MacGyver'd version of the stock cable. I recently just finished fixing that. With the assistance of sharp eyes from other smart Mustang people, I figured out that the brakes have been upgraded too...sort of. The front rotors look to be the same size as stock but are drilled.
IMG-6663.JPG

The rear brakes have also been upgraded to disc brakes (here's pics of the inside but I haven't taken rear wheels off to look at the rotors yet).
IMG-0502.jpg IMG-0500.jpg

The car also has a Wilwood adjustable proportioning valve.
IMG-7412.jpg

It looks like a previous owner did the 3-2 conversion but the master cylinder looks very similar to stock. Someone pointed out that it looks like my brake lines from the master cylinder to the factory proportioning valve are copper. A few folks on other forums have chimed in that copper lines are not good and that I should try to address this asap. Initially I was looking at purchasing the 1993 Cobra Master Cylinder, which appears to be somewhat of an upgrade, and can fit on 87-93 GT Mustangs without major modifications. The problem I'm anticipating though, is the Maximum Motorsports master cylinder installation kit doesn't appear to include the front port brake line, which I obviously need to replace too. I'm not seeing any front port lines available on MM or LMR.

I also looked at the 94-95 Cobra MC on MM and it looks like their installation kit does include a complete line kit. Anyone have thoughts on the 94-95 Cobra MC for a mostly stock 1988 GT? One person said that they thought this was typically for people upgrading to 13 inch Cobra front brakes.
IMG-0355.jpg

I ended up ordering the 1993 Cobra master cylinder and Maximum Motorsports installation kit, thinking that I can just see about putting together a front port brake line or getting one from a parts shop. So basically my question after this long-winded thread is, could I just do the 94-95 Cobra master cylinder without any significant adjustments? Or should I stick with my plan on going with the 1993? If I could do the 94-95 and more complete installation kit, I am fine with trying to change my order so I don't have to piece together a front port brake line. Thanks.
 

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Late to your saga. I watched a couple of your vids in the other thread. What I have to say is based upon years of brake iteration and suspension upgrades to my own 86 Mustang. What follows may be old news or not, but should be helpful to others if not to you.

It appears you have a rear disc conversion based upon the 93 Cobra/TB Turbo Coupe or the SSBC conversion. I converted to rear discs using the SSBC kit circa '91 or so, way before 4-wheel discs on Fox Mustangs was the norm. This kit uses a 45mm rear caliper piston and a 10-11/64" (call it 10.25") vented 4-lug disc. Based upon the pics in this thread you appear to have a stock 66mm front caliper and 11" drilled/vented disc. In combination with the rear disc conversion this is nearly identical to what Ford speced for the 93 Cobra. Thus, brake torque bias with this set-up is F66%/R34% for a Fox chassis. Swapping to the 93 Cobra MC was good since it is a better match to the front/rear caliper sizes. Matching MC size to calipers helps to determine pedal feel. Too big a MC and you get a stiff/short pedal; too small and you get a soft/long pedal, assuming the calipers stay the same.

When swapping a different MC from stock, you have to make certain that the threaded rod is the proper length to fully engage the MC without damaging the internals. Maximum Motorsport has a guide imbedded in their MC installation instructions.

You need to gut the stock prop valve and plug it with a special plug. This essentially eliminates the factory stock knee-point setting which is meant to prevent rear brake lock before front brake lock. But you have to insert a manual prop valve in the rear brake line to reestablish a new knee-point appropriate for rear discs. Keep in mind that the prop valve does not adjust brake torque balance. Brake torque bias is set by the caliper and rotors sizes used. The valve's only function is to set a knee-point on the rear brake pressure slope where the pressure decreases in an emergency braking situation to prevent rear lock before front lock. Otherwise, the valve is a pass-through and has no influence on rear pressure. Most enthusiasts get this wrong and believe the prop valve can correct mismatched brake torque bias. It can't.

To set the rear knee-point, you have to perform stop tests. Start with the prop valve all the way out then in 4 turns; I marked the knob with a red paint stripe to help with counts. To avoid severely flat-spotting tires, I suggest going to an empty parking lot, on a dry day, get a little speed up and then stand on the brakes and observe whether the fronts lock first or not. It helps to have an outside observer but I've done it solo. Adjust the manual prop valve in or out depending on outcome. Try to get the rears to just lock after the fronts. This may be difficult to achieve but at least have the fronts lock first even if you can't get the rears to lock. Once set, leave it alone until some other changes are made. Usually, anything short of a major suspension upgrade say to a T/A or after-market k-member, will not have an impact on the knee-point setting.

Good luck and hope it goes well for you.
 

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Discussion Starter #3
Late to your saga. I watched a couple of your vids in the other thread. What I have to say is based upon years of brake iteration and suspension upgrades to my own 86 Mustang. What follows may be old news or not, but should be helpful to others if not to you.

It appears you have a rear disc conversion based upon the 93 Cobra/TB Turbo Coupe or the SSBC conversion. I converted to rear discs using the SSBC kit circa '91 or so, way before 4-wheel discs on Fox Mustangs was the norm. This kit uses a 45mm rear caliper piston and a 10-11/64" (call it 10.25") vented 4-lug disc. Based upon the pics in this thread you appear to have a stock 66mm front caliper and 11" drilled/vented disc. In combination with the rear disc conversion this is nearly identical to what Ford speced for the 93 Cobra. Thus, brake torque bias with this set-up is F66%/R34% for a Fox chassis. Swapping to the 93 Cobra MC was good since it is a better match to the front/rear caliper sizes. Matching MC size to calipers helps to determine pedal feel. Too big a MC and you get a stiff/short pedal; too small and you get a soft/long pedal, assuming the calipers stay the same.

When swapping a different MC from stock, you have to make certain that the threaded rod is the proper length to fully engage the MC without damaging the internals. Maximum Motorsport has a guide imbedded in their MC installation instructions.

You need to gut the stock prop valve and plug it with a special plug. This essentially eliminates the factory stock knee-point setting which is meant to prevent rear brake lock before front brake lock. But you have to insert a manual prop valve in the rear brake line to reestablish a new knee-point appropriate for rear discs. Keep in mind that the prop valve does not adjust brake torque balance. Brake torque bias is set by the caliper and rotors sizes used. The valve's only function is to set a knee-point on the rear brake pressure slope where the pressure decreases in an emergency braking situation to prevent rear lock before front lock. Otherwise, the valve is a pass-through and has no influence on rear pressure. Most enthusiasts get this wrong and believe the prop valve can correct mismatched brake torque bias. It can't.

To set the rear knee-point, you have to perform stop tests. Start with the prop valve all the way out then in 4 turns; I marked the knob with a red paint stripe to help with counts. To avoid severely flat-spotting tires, I suggest going to an empty parking lot, on a dry day, get a little speed up and then stand on the brakes and observe whether the fronts lock first or not. It helps to have an outside observer but I've done it solo. Adjust the manual prop valve in or out depending on outcome. Try to get the rears to just lock after the fronts. This may be difficult to achieve but at least have the fronts lock first even if you can't get the rears to lock. Once set, leave it alone until some other changes are made. Usually, anything short of a major suspension upgrade say to a T/A or after-market k-member, will not have an impact on the knee-point setting.

Good luck and hope it goes well for you.
Thanks for your input. I managed to figure most of this out through trial and error + the help of folks on other forums. I recorded the install and challenges over the course of three or four videos. I forgot to update this particular thread but I do have another progress thread going here:

 
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