CNC Master Cyl + Vac Assist?

[bwwooster]

Hi- I'm having the front brake lock-up problem. I'm using the SN95 Cobra master cylinder, stock Fox booster (SVO Crate 302- does the E303 cam affect the amount of vacuum to the booster? hmmm...), stock Fox front brakes and stock T-bird IRS disc at the back, Porterfield R4-S pads all around. I started with a Wilwood proportioning valve on the rear circuit, but now it's backed all the way off and having just freewheeled the car down a long grade, the rear discs were barely above ambient temp while the fronts were hot.

In searching all the threads, it looks like the ideal, ultimate solution is really to go to a dual master cylinder with a bias bar and I read somewhere that the CNC dual unit can be used on the Mustang booster.

FACT

OR

FICTION?

Also, where can I get one of these things? CNC's site's got no info on buying them.

I should also mention that I've just picked up a pair of '95 SN95 spindles and am planning to do the Borden/Whitby SAI mod and might want to go to the 13" discs and PBR calipers 'cos I'd like to use it on track days.

Thanks!

John

P.S. I've also got an odd thing going with the action of the brake pedal ever since I added the booster: it takes a pretty good swing before I get ANY resistance (at least an inch at the foot pad) and then it's hard as a rock with little give, like it's against a mechanical stop. The only other cars with power brakes that I've driven and have taken that much motion were early ABS cars (a Mercedes C240 and a VW Corrado) from the mid '90s and when the brakes caught, they still had a not-fully-bled squishiness. Every other car with power brakes has only taken a fraction of that distance to start working. Maybe this should be a separate thread...

Anyway, there it is.

[Bob Cowan]

Mike Forte sells the dual MC with booster. If you don't see it on his site, call him and ask.

Even when fully open, the valve still blocks some of the pressure. You're better off removing it. You might consider installing it in the front brake line.

[CraigS]

Also check w/ Porterfield about a grippier rear pad compound. Most of the brake manufacturers refuse to publish coefficient of friction numbers for their pads so all you can do is call. Be sure to ask about grip at ambient temps. You don't want a pad that needs warming to work.

[Dan Babb]

Make sure the brake pedal is fully releasing from the MC. If there's any pressure at all on the MC, it will push the front brakes in just enough for them to heat up and ultimately lock while driving.

You can also check the snap ring that holds the piston in the MC. If that ring is twisted (happened to me), it will not allow the piston to fully release and you have a problem. Not sure if the MC you have is like the Jeep MC I used, but it's worth checking.

[rich grsc]

You already have too much front brake, why would you think you need 13" fronts?

[bwwooster]

I'm not sure I do, but I want to do the SAI thing, which requires the SN95 thing, which gives the option of the 13" Cobra brakes, which would have more heat capacity for repeated runs into turns 2, 5, 8 and 11 at Laguna Seca.

The other problem, which I've just discovered, is that my 16" Team III wheels have a really flat web from the lug flange out to the rim and contact that bar on the stock floating caliper of the SN95 single piston brakes. They just barely clear the same part of the Fox brakes. Hmmm. Using wheel spacers might work, but would require longer lug studs which would be a slight PITA when switching back and forth between the nice looking Team IIIs and the 17" wheels I'm putting stickier tires on for track days.

Does anyone know whether that bar (it's part of the solidly- mounted bracket that the floating part of the caliper rides on) of the Cobra caliper (it's the straight one) stands proud of the plain of the wheel mounting flange, and by how much?

Thanks!
John