Brakes The Ultimate 4-lug/5-lug Brake Conversion/upgrade Thread

NightFury · Aug 3, 2015

Mustang5L5 said:
Which prop valve do you have?

I believe the wilwood style valve is fully open all the way screwed in.

Does your pedal get rock hard and barely budge with the engine off?

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It's the Ford Motorsport prop valve (same as Wilwood AFAIK). Yes with engine off, pedal is very stiff after pumping 1 or 2 times.

Mustang5L5 · Aug 3, 2015

Rebleed it with knob fully turned in.

Your setup is pretty common, so you shouldn't have soft pedal issues. It's the same setup as a 99+ v6 stang

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NightFury · Aug 10, 2015

Mustang5L5 said:
Rebleed it with knob fully turned in.

Your setup is pretty common, so you shouldn't have soft pedal issues. It's the same setup as a 99+ v6 stang

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Re-bled, no change. Pedal still bottoms out. It acts like there isn't enough fluid being pushed through the lines but that wouldn't make much sense if the 93 Cobra and 99 V6 MCs are identical.

90sickfox · Aug 10, 2015

I had the same problem on my car...changed to a newer master cylinder, booster, and prop valve. No problems...can actually lock brakes without bad nose dive. I run cobra front and rear IRS brakes. I've known people that were successful doing it that way...but it didn't work for me :shrug

Mustang5L5 · Aug 10, 2015

NightFury said:
Re-bled, no change. Pedal still bottoms out. It acts like there isn't enough fluid being pushed through the lines but that wouldn't make much sense if the 93 Cobra and 99 V6 MCs are identical.

Is the MC new? Are you 100% sure it's a 1993 cobra MC? It required a 3-2 conversion right?

Your combo is the same as the 99V6 and plenty of guys have the same setup and it works great. So I wonder if there is a mechanical issue with your setup. Bad MC? Leak on one of the calipers? Etc.

NightFury · Aug 10, 2015

Mustang5L5 said:
Is the MC new? Are you 100% sure it's a 1993 cobra MC? It required a 3-2 conversion right?

Your combo is the same as the 99V6 and plenty of guys have the same setup and it works great. So I wonder if there is a mechanical issue with your setup. Bad MC? Leak on one of the calipers? Etc.

The MC looked new and in fine working order. The box said 93 Cobra. I got it from Summit. The booster was a reman but if anything, it's working too well. When I bled the brakes, there was a consistent solid stream of fluid coming from the bleeder valves, so the MC seems like it's pushing fluid through the lines just fine. I've also checked all the connections after driving it and there are no wet areas around any of the calipers. Pad wear seems to be even too, so I'm presuming the pistons aren't sticking. I gutted the stock prop valve, ran all new stainless braided hoses. Pads are Hawk ceramic. Are there any tests I can do that would help narrow down my possible problem areas? Maybe when it's under the pressure the MC is leaking back into the resivoir? A head scratcher for sure. Thanks again for brain storming with me.

Mustang5L5 · Aug 10, 2015

Well, if the system is 100% working right, then there is one thing you can try.

The 1993 Cobra MC is a 1" bore. The next size up is the 94-95 Mustang GT MC which is a 1 1/16" bore. Fortunately, the port sizes are the EXACT SAME as the 1993 Cobra, so is should be a direct plug and play without changing any of the lines. If it isn't, then the MC you installed was not a 1993 Cobra MC....however that's a long shot as I can't see Summit mixing it up.

Since this MC is regular production stuff, it should be cheaper, with even used versions in the $20 range as it's so common.

Now, ideally, the 1993 cobra should work fine, but since you are claming the pedal is soft, and travel is long, then you might want to try a larger bore MC as this will push more fluid. As a result, your pedal should firm up, and stroke should shorten.

NightFury · Aug 11, 2015

So thinking back on it, I think the MC I got was a Cardone unit, which means it would have been a remanufactured piece. Based on the way I'm able to push the pedal to the floor, with no visible external leaks, I'm waging that I got a bad MC.

So now I'm going to purchase a new Centric unit, and the prices are the same between the 93 Cobra and the 94/95 GT units. Will I notice much of a difference with that extra 1/16th of an inch or should I just re-try another Cobra unit?

Mustang5L5 · Aug 12, 2015

1/16" inch is enough to notice a difference. Whether or not that difference is "good" or "bad" is up to you.

Ideally, I try to recommend sticking to as close to a factory brake system as possible. With that said, the 1" cobra MC would give you the same system as the 99-04 V6.

I know a lot of 94-95 guys swap on the 99+ PBR's and report a firm pedal and difficulty getting wheels to lock (not that that's a measure of brake performance) while others like the feel.

With my Cobra brakes, I tried the 94-95 MC for a while and it was way too firm. Felt like I didn't have much braking power. Swapped to the 93 cobra MC and that 1/16" reduction made a huge difference in feel.

NightFury · Aug 14, 2015

So the bad news is, I replaced the MC with a new unit, same problem. I guess the good news is, I've narrowed the problem down to the calipers (I mean, what else could it be, right?)

But just so I can understand it better, how would a bad caliper cause a soft pedal? I feel like if it were not holding pressure there would be evidence all over my garage floor. Does it leak back inside of itself somehow? Thanks.

Shakerhood · Aug 14, 2015

I didn't want to read back through the previous pages, but do you have the Calipers on the correct sides, Bleeders on top?

NightFury · Aug 15, 2015

Yes, I'm pretty sure I double checked that when I installed them at least three or four times haha.

What seems strange though, is if I had a bad caliper, wouldn't the car pull to the side of the stronger caliper? My system feels weak all the way around. Could there be an issue with the gutted stock prop valve?

Mustang5L5 · Aug 15, 2015

I've seen this with bad rear calipers. The car won't pull in this case. What happens is the slide pins of piston seizes and the calipers movement is restricted. The soft pedal comes from that particular caliper not moving properly.

But you said this was a new setup right?

This one has me scratching my head. Even if the MC wasn't sized correctly your pedal shouldn't go to the floor.

Did you use a standard 3-2 conversion to install the MC?

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NightFury · Aug 15, 2015

Well, new to me. I got the front's off ebay from a car with less than 80,000 miles and I pulled the rears off a car at the pick-n-pull. I compressed all four pistons myself, inspected the outer seals, greased the slide pins and everything "looked ok". I attempted to get them all rebuilt with new seals but no brake shop in town would touch them. Liability reasons I guess. So the possibility certainly exists that one or more of the calipers are somehow bad. I can see brake pad wear on all four rotors, so each caliper is at least getting some pressure applied to it. I wonder which ones I should replace first...

And yes about the 3-2 conversion. I got the Maximum Motorsports kit.

NightFury · Aug 15, 2015

Side note... I noticed the 99-02 PBRs are sometimes listed separately from the 03-04 PBRs. Is there much of a difference between the years?

Chuckman · Aug 15, 2015

NightFury said:
Side note... I noticed the 99-02 PBRs are sometimes listed separately from the 03-04 PBRs. Is there much of a difference between the years?

Thread pitch on the banjo bolt changed from fine to coarse, and the cobra pbrs got 2mm bigger pistons

Mustang5L5 · Aug 16, 2015

You have the GT calipers right? 10.8" rotors? If so, the difference is just banjo bolt pitch threads.

Cobra PBRs break down as 94-98 and 99-04 with banjo pitch and piston size as differences.

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NightFury · Aug 16, 2015

So I found this article...

Tech Center - Causes of a Low Brake Pedal

It mentions that calipers with ratcheting ebrakes like ours may need to be adjusted. Is that something I should look into? If so, how do I adjust it out?

Mustang5L5 · Aug 18, 2015

The rear calipers are a racheting style. The issue with these is that if they sit for a while (junkyard) or are not used and the brake fluid is old and fluid filled, the mechanism tends to rust and causes issues with piston extension.

How well does your parking brake work? If you go for a drive and do a few 35-0 stops, are the rear rotors warm compared to the fronts? I'd say use an IR gun here, but if you don't have one, gently try and touch the rotor surface.

I hate to suggest replacing parts as i'm not there to inspect and they may not be bad, but if you suspect the rear calipers may not be of good quality, it might be worth it to buy a couple replacements (rockauto.com) and use those as cores. Junkyard parts can def be suspect...especially if they sat for a while. But then again, it's possibly they are fine so check them out first. My rear calipers are junkyard specials, that sat for 4-5 years after I bought them before I installed. Mine work fine. So it's hit or miss if they are bad or not.

Also, does "double pumping" the brake pedal while stopping build the pedal up and give better brake response?

Carl H · Aug 27, 2015

I hoping I didn't miss a post in here somewhere... I'm having a similar soft/hits the floor pedal problem. 93 4cyl booster(stock), wildwood prop valve(p/n #260-11179), 96 V6 m/c, 96 front calipers, TC rear calipers. From what I read here and doing the math I come up with a 17.52 ratio.
Fronts: 6842mm^2
Rear: 3180mm^2
MC: 572mm^2

My parking brake is adjusted but doesn't want to let the piston move back to original position so I quit using it for awhile. The front calipers seem to work ok (looking at wear marks on the rotor), the rears are sketchy at times but I know there is zero air in the system. Last night I was trying to break in my new QTP's and the line lock wouldn't hold and I literally had the pedal on the floor trying to keep the car from pushing.

I am considering stepping up to the SVO m/c, thoughts?

Brakes The Ultimate 4-lug/5-lug Brake Conversion/upgrade Thread

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