Did 5 Lug/4 Disc Swap, Brakes Are Weak

Jason 302 · Feb 3, 2016

Finally did the 5 lug/4 wheel disc swap on my car. Used the following parts....

94/95 spindles
JM Stainless brake lines up front
99-04 PBR dual piston calipers up front
Gutted and plugged stock proportioning valve
Ford Racing adjustable proportioning valve
94/95 GT Master Cylinder
Stock fox vacuum booster
StopTech "Street" Pads up front
Brand new stock 94+ GT rotors on all four corners
McClure line lock kit
Moser 31spline 5 lug rear axles
Maximum Motorsports rear brake line adapter kit
94+ GT calipers in the rear
Ceramic rear pads
Proportioning valve turned in 4 turns in from all out

So far the brakes are extremely underwhelming, takes a TON of pedal pressure in order to stop the car in a hurry, and even then it's not doing so very well. The factory brakes did a better job, if that tells you anything. Do I need to give the StopTech pads some time to seat? Is the stock brake booster way too under powered for 4 wheel disc? Should I bypass the line lock kit, it worked fine with the old setup. Really disappointed thus far with the swap (other than the looks). Some pics for reference....

jrichker · Feb 4, 2016

Is the pedal spongy or do the brakes pump up?

madspeed · Feb 4, 2016

was the master cylinder properly bech bled before install?

elarm1 · Feb 4, 2016

The problem is your STOCK booster. I just replaced mine with a Cobra unit. No more sponge brakes

Mustang5L5 · Feb 4, 2016

Your issue is due to incorrect MC/Booster.

Your brake setup came factory (on the 99-04 V6 Mustang w/vacuum assist brakes) with the larger booster and a 1" bore MC. By sticking with the stock fox booster and going with a 1 1/16" bore MC, you've created a situation where you need a LOT more pedal effort to stop.

The 94-95 GT MC is ideal only for the 66mm single piston calipers. The 99-04 calipers have slightly less surface area so your MC needs to be a bit smaller.

Long story short:

1993 Cobra 1" bore MC
94-95, 99-04 booster or 1993 Cobra booster

Jason 302 · Feb 4, 2016

jrichker said:
Is the pedal spongy or do the brakes pump up?

Pedal feels firm about half way down

madspeed said:
was the master cylinder properly bech bled before install?

Yes

elarm1 said:
The problem is your STOCK booster. I just replaced mine with a Cobra unit. No more sponge brakes

Thank you, I was considering that

Mustang5L5 said:
Your issue is due to incorrect MC/Booster.

Your brake setup came factory with the larger booster and a 1" bore MC. By sticking with the stock fox booster and going with a 1 1/16" bore MC, you've created a situation where you need a LOT more pedal effort to stop.

The 94-95 GT MC is ideal only for the 66mm single piston calipers. The 99-04 calipers have slightly less surface area so your MC needs to be a bit smaller.

Long story short:

1993 Cobra 1" bore MC
94-95, 99-04 booster or 1993 Cobra booster

This is what I was afraid of. I knew 94/95 had single piston calipers, but I had already bought the master cylinder a few years back. I was hoping I could make it work. Looks like I need to spend some more $$$$

Jason 302 · Feb 4, 2016

Thanks everyone for your replies. I'm going to order a 93 Cobra booster and M/C here next month and see where that gets me. I'll give an update and see how it works out.

BTW it's good to be back, I haven't been on here much the last several years, mustang took a back seat to motorcycle racing. All that nonsense is behind me now lol

jrichker · Feb 4, 2016

Jason 302 said:
Thanks everyone for your replies. I'm going to order a 93 Cobra booster and M/C here next month and see where that gets me. I'll give an update and see how it works out.

BTW it's good to be back, I haven't been on here much the last several years, mustang took a back seat to motorcycle racing. All that nonsense is behind me now lol

The love affair with keeping all the parts of your human body must have caught up with you again...

2000xp8 · Feb 4, 2016

I ran similar brakes for years and they were just fine without the booster.
Only difference would be I used the 93 cobra MC.

elarm1 · Feb 4, 2016

@2000xp8 i was cool with just puttinh around but the panic stops were down right dangerous for me. I almost rear ended my buddy on his bagger and then I vowed to fix them then

sent from my butt pocket

Jason 302 · Feb 4, 2016

jrichker said:
The love affair with keeping all the parts of your human body must have caught up with you again...

Never got hurt too bad luckily, just a concussion one time.

elarm1 said:
@2000xp8 i was cool with just puttinh around but the panic stops were down right dangerous for me. I almost rear ended my buddy on his bagger and then I vowed to fix them then

sent from my butt pocket

That's how mine are, they don't feel too bad initially, and do the job for normal driving, but when it gets serious it gets scary.

90sickfox · Feb 4, 2016

I took the booster MC and prop valve off a 98 v6 for my rear IRS and cobra front brakes. Works well.... Way better than stock.

Mustang5L5 · Feb 5, 2016

90sickfox said:
I took the booster MC and prop valve off a 98 v6 for my rear IRS and cobra front brakes. Works well.... Way better than stock.

Same issue as OP though. That MC is 1/16" too large. While they may work great, they would work even better with the 1" bore MC.

Mustang5L5 · Feb 5, 2016

2000xp8 said:
I ran similar brakes for years and they were just fine without the booster.
Only difference would be I used the 93 cobra MC.

I'd rank the MC more important than the booster. Some guys have big quad and can modulate the brakes well with the smaller boosters, but an MC bore reduction of 1/16" makes a huge difference in pedal feel.

If the OP was to pick one of the two changes I'd suggested only, it would be to do the MC first, and see how he likes it then. If still not enough power, then do the booster.

Ideally, I tend to recommend running as close to factory setup as possible...with a few exceptions here and there.

Gearbanger 101 · Feb 5, 2016

Curious, would you guys recommend just the 1" MC if a person was simply doing the front brakes only and leaving the drum brakes in the rear?

I think I might make a partial brake upgrade part of my project this summer, but haven't sourced rear disc components yet.

Mustang5L5 · Feb 5, 2016

Gearbanger 101 said:
Curious, would you guys recommend just the 1" MC if a person was simply doing the front brakes only and leaving the drum brakes in the rear? .

Depends on what the front brakes are. In general though you do not want to run a MC designed for 4 wheel disk brakes with rear drums. Rear cylinders don't need much fluid to operate, so swapping in a 4wdisk MC can leave you with a hard pedal. There really isn't an ideal MC for only swapping the fronts, and leaving the stock MC is usually the best solution.

The recommendations are based on mathematical ratios of the Master cylinder piston area vs the slave cylinder (calipers) piston area. There are engineering reasons for the recommendations I've made. I don't want anyone to think I just pick them out of a hat.

Refer to

http://www.stangnet.com/mustang-for...onversion-upgrade-thread.801400/#post-8075734

Jason 302 · Feb 5, 2016

Mustang5L5 said:
I'd rank the MC more important than the booster. Some guys have big quad and can modulate the brakes well with the smaller boosters, but an MC bore reduction of 1/16" makes a huge difference in pedal feel.

If the OP was to pick one of the two changes I'd suggested only, it would be to do the MC first, and see how he likes it then. If still not enough power, then do the booster.

Ideally, I tend to recommend running as close to factory setup as possible...with a few exceptions here and there.

I'm probably just going to do both and be done with it. I wish I had the parts when I had it all tore apart the first time, but oh well.

mikestang63 · Feb 5, 2016

I'd try the cobra MC first. I did my booster last year and i'm not that young or limber. It was a major PITA laying upside down under the dash to get that damn upper left bolt and the rod off the pedal.

Jason 302 · Feb 5, 2016

mikestang63 said:
I'd try the cobra MC first. I did my booster last year and i'm not that young or limber. It was a major PITA laying upside down under the dash to get that damn upper left bolt and the rod off the pedal.

Ratchet wrench make it any easier?

Grn92LX · Feb 6, 2016

^ thats how I installed mine.

Did 5 Lug/4 Disc Swap, Brakes Are Weak

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