Brakes Trouble W/ 93 Cobra Booster In 90, Brake Switch Interference

spoulson

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So, I decided to upgrade the brake booster to a '93 Cobra booster to see if it'd fix my weak brakes. Weak, as in, I can't lock up the brakes at any speed.

I've had weak brakes in my car since I bought it. It's pretty modified, so here's a quick background: 5.0 with HCI, Baer sport 4-wheel disc brake upgrade, SN95 master cylinder, modified combination valve, adj prop valve to rear.

What I've checked so far: pads/rotors are in good shape. Engine idles with about 12mmHg. Can't identify any significant vacuum leaks. I put a line pressure gauge on the front and rear calipers. With the engine running and after a few revs to power up the booster, I could only push about 1100psi front and rear when mashing the pedal. The stock booster can hold at least 20mmHg when using a hand vacuum pump.

I went the route of the booster upgrade to see if it'd counteract the low vacuum, as some research pointed out some people were able to get normal brake capacity after this upgrade.

Now the problem:
I got the booster bolted in. However, the brake switch now gets in the way of the steering column brace and wants to crush the connector on release of the pedal. I tried getting a SN95 brake switch, since the connector mounts to the side, instead of behind. It, too, gets in the way of the brace in a different spot.

What could I be missing? Do I need a spacer on the booster?
 
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bird_dog0347

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Pics of the issue? I just swapped the stock booster and master (from a 92 4 banger) in my car to a 95 booster and master in order to match the 4 wheel discs from a 95. The booster was a PITA to install, but I didn't have any interference issues other than the strut tower where I had to beat the crap out of it. I am still working on bleeding them up so no final word on outcome yet. Just waiting on my speed bleeders to get here as the ones I have suck and I can't get it to bleed up perfectly. It should be done by the weekend though.
 

Mustang5L5

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I need pics. I'm not picturing this.

I put an SN95 booster and 93 Cobra MC without issues. Don't recall anything hitting the steering
 

jrichker

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The booster swap is a band-aid for a mismatch of the master cylinder and the wheel cylinders. With the proper ratio of master cylinder piston area to wheel cylinder piston area, the stock booster should work fine. The only limitation is the amount of pedal stroke you need and are willing to put up with. Post the wheel cylinder piston sizes and maybe Mustang5L5 will be kind enough to do the math to show you how to use a different master cylinder to match the wheel cylinders.

Mustang5L5
 

spoulson

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Here's a pic of the switch on the brake pedal peg/booster rod without the wire connector attached, looking straight up from the floor, firewall directly to the right. To the left is the steering column brace that switch is hanging just above.
Hy5JKl.jpg


I attach the wire connector and the connector rests on top of the brace. As I press the brake pedal with my hand, it slides along the brace until it just barely falls off the edge. In past tests, it gets snagged and stresses the brittle plastic and jams the pedal. Although, this time it snagged only slightly, but didn't jam the pedal.
8x8zCl.jpg


Here is the opposite view, looking towards the firewall where you can see the cap on the wire connector is resting on the brace. When the pedal jams against it, that plastic cover snaps off. On one side of this piece, the plastic had actually broken a snap and it doesn't stay on very firmly anymore.
TI9Rll.jpg


I should note that the braking system is not fully bled yet. I noticed the problem before I reinstalled the master. I installed it (not bled yet) and the pedal travel is a little more limited, but still jams or snags the switch against the brace.

I'm concerned this is a big safety issue where I won't be able to release the brake pedal fully. Is the switch supposed to ride along the brace or does it hang higher and out of the way?

Here's an illustration I found in another thread:
stoplampswitchpage2.jpg
 

spoulson

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The booster swap is a band-aid for a mismatch of the master cylinder and the wheel cylinders. With the proper ratio of master cylinder piston area to wheel cylinder piston area, the stock booster should work fine. The only limitation is the amount of pedal stroke you need and are willing to put up with. Post the wheel cylinder piston sizes and maybe Mustang5L5 will be kind enough to do the math to show you how to use a different master cylinder to match the wheel cylinders.
You may be right, but I believe the limiting factor here is low engine vacuum. The brake setup is more specifically a Baer brake setup that uses C4 Corvette front/rear calipers. I don't know bore sizes, but it's a kit made for the Fox bodies. I spoke with a tech at Baer and after explaining my situation he was certain low vacuum was the culprit. I can't be sure this booster upgrade will really work, but I felt it was worth a try after reading a little about increasing booster capacity. example: http://forums.corral.net/forums/general-mustang-tech/1146858-people-low-vacuum-cobra-brakes.html
 

jrichker

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You may be right, but I believe the limiting factor here is low engine vacuum. The brake setup is more specifically a Baer brake setup that uses C4 Corvette front/rear calipers. I don't know bore sizes, but it's a kit made for the Fox bodies. I spoke with a tech at Baer and after explaining my situation he was certain low vacuum was the culprit. I can't be sure this booster upgrade will really work, but I felt it was worth a try after reading a little about increasing booster capacity. example: http://forums.corral.net/forums/general-mustang-tech/1146858-people-low-vacuum-cobra-brakes.html
Here's Google rabbit you can chase to find the piston sizes...
http://www.google.com/search?q=C4+C...-us&ie=UTF-8&oe=UTF-8&startIndex=&startPage=1
 

RangerJoe

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My switch hit too. I took a zip tie a tied it up around a wiring bundle. Not much, just a fraction of an inch to let it clear, and it is tied very loose, so its not rubbing anything. My switch was hitting the same area and keeping my brake lights on all the time.

Joe
 

Mustang5L5

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Which MC are you using? the 94-95 GT/V6 MC?

And you have the Baer kit correct? So that should be 94-98 Cobra style calipers up front and C4 rear calipers.

So 38mm x 2 pistons up front for each side, and I believe the rears are 54mm pistons? I'll have to google that.

EDIT: Rockauto.com shows 40mm rear calipers, so very similar to the SN95 38mm's.
 

spoulson

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I believe it is an SN95 MC but I don't know how to identify it. I once had a 93 Cobra MC that I decided not to swap in. They both look exactly the same, except the 93 Cobra MC had no code cast into the bottom of the unit. The SN95 had some numbers, but I never wrote them down while I had it out.

Ok, well I thought the Baer kit gave you C4's front/rear. In any case, from what I've read the Baer street kit is considered pretty equivalent to the M-2300K kit brakes. How do you calculate whether a MC matches the calipers?
 

Mustang5L5

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The Baer kit pretty much is C4 brakes, but the front C4 brakes are the same as the 94-98 Cobra caliper, except for cosmetic casting differences. Internally, same 38mm pistons as the Cobra brakes.

Out back, the C4 uses 40mm pistons by my googling. The Cobra uses 38mm pistons.

SO basically it's a 94-98 Cobra setup.


94-98 Cobra's came with the larger booster (identical to the 93 cobra booster you installed except for different bolt stud pattern) and a 15/16" MC. I remember years back on another Mustang site talking to an engineer who worked on the brakes on the Mustang in 1994. He claimed the original MC sizing was a 1" bore MC, but at the last minute, they dropped the bore size to 15/16" due to some issues with brake performance on cold pads.

The GT/V6 uses a 1 1/16" MC which is much too large for the Cobra brakes (or Baer in your case). You will find you will have trouble getting good braking power with this setup. I'd at least step down to a 1" 1993 Cobra MC or ideally the 94-95 Cobra MC. The 1993 Cobra MC will be a direct swap while the 94/95 will take a little bit of replumbing since the fittings are reversed.
 

spoulson

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How can I measure bore size on a MC? Do I have to disassemble it?

Thing is, I don't know exactly which one I have. I'm not sure if Baer provided this MC or if the original owner's mechanic sourced it. Some Google searching turned up a thread saying Baer provides a 15/16" MC. I saw nothing in the Baer instructions about the master cylinder replacement. I don't know how to identify this from the exterior, either. Ideas?
 

spoulson

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I was able to complete the install over the weekend and after a quick drive, I can finally say I have decent braking power! For me, the 93 booster was a necessity to complement the big cam and Baer sport brake kit. Pedal effort is normal and I can finally lock up the brakes on a panic stop, which I couldn't do before.

After some test driving I could not duplicate the brake switch interference issue I originally reported. I hope that it doesn't recur, but for now it seems by design to want to slide along that brace but normal pedal travel doesn't normally go so far as to let it fall off and get jammed. I noticed this jamming while the MC was not installed and again when installed, but not bled. Now that it's fully bled, it doesn't seem to be an issue.

So while I'm not certain which master cylinder I have, I do know that it's metric fittings and the larger port is in the rear. I can't find any references that tells me which models got the larger port in front or rear.

And I will agree, the booster install was a huge PITA. I spent hours removing parts to make room, trimming mounting studs, slotting firewall holes, trying to fit and refit the booster in so as to avoid shock tower modifications. But, I ultimately put the minisledge to work and then it dropped right in. It seems that all these steps were necessary for my car.

Thanks for the advice, all.
 
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