Fox Brake pedal too stiff

[keel]

I was doing some spirited driving over the weekend and realized that in order to get full braking power I need to put way too much effort into the pedal. To be clear, I don't think there is anything mechanically compromised with the braking system, just something wrong with the way I set it up.

I'm running SN95 Cobra brakes on all four corners, gutted OEM proportioning valve and a Wilwood adjustable to manage brake bias, and a 1993 Cobra master cylinder with a Fox V8 booster. I've done some research and it seems like a '93 Cobra booster is what I need to get the pedal feel I'm after, but I also wonder if I need to go to a different master cylinder too. Looking for some opinions for this combo and how to proceed so I can get it right without messing around too much.

 

[AeroCoupe]

Cobra MC is the preferred MC with the 94-04 cobra brakes on our cars.

Several folks have run the Fox booster with success so I would not just say they don’t work. Most run the 1993 Cobra booster as the studs on it fit the existing g firewall holes and it does provide more boost than the Fox unit can.

Based on my experience I would run the 1993 MC and booster. Stick with known brands of boosters.

 

[Mustang5L5]

Which generation Cobra brakes do you have up front?

If you have the 94-98 versions, you'd really want the 94-95 Cobra 15/16" MC. (they can be used with the 1" 93 cobra MC but first stop on cold pads might be a little lacking)

If you have the 99-04 versions, the 93 Cobra 1" MC is the right one.

1993 Cobra booster should be run with both.

Easy way to tell is to cut the wheel and look behind at the piston bores. 94-98 are ribbed, 99-04 are smooth

 

[keel]

I'll have to take a look. What's the difference between them internally?

 

[86HO5.0]

94-98 are 38mm. 99-04 are 40.5mm IIRC.

The 94-95 Cobra M/C (15/16") works great with the 38mm calipers and stock Fox booster. The Cobra M/C is marginally larger than the Fox M/C, roughly 1/16".

 

[Mustang5L5]

Off-topic, but information that deserves to be recorded.

Years ago I was conversing with a Ford engineer on corral who did the brake system for the 1994 Cobra. This is the story he told me regarding the development of the 1994 Cobra brakes. Originally the setup came with a 1" bore MC. Well apparently on cold pads and such, the system was unable to lock up the brakes or at least approach lockup until some heat was generated in the pads. They were demonstrating the car to execs and were underwhelmed by the initial braking performance on the first few stops on the largest braking system Ford had ever put on a mass production car. SO the MC was changed to the 15/16" bore.

 

[keel]

Would a 94-95 Cobra MC work with both types of brake calipers? Or would it be over assisted for the later calipers with the larger piston?

I figure I'm going to have to remove the MC anyway to install the booster and if I had to choose then I would rather the braking power be on the stronger side. Especially after reading what @Mustang5L5 said about the development of the '94 Cobra.

 

[86HO5.0]

The 94-95 MC would be a better match for the 94-98 Spec Cobra brakes.

I'm running the 94-95 MC, stock 86 booster with 94-98 spec Cobra brakes FR and Rear. Braking power is VERY strong. I've also deleted the stock combination valve and installed a FRPP adjustable proportioning valve where the stock combination valve was located. Excellent setup, highly recommended.

 

[AeroCoupe]

@86HO5.0 - just a vocabulary thing but if you deleted the brass block below the MC you deleted the combination valve. That brass block houses the proportioning valve and shuttle valve hence the name combination valve. Again, just wanted to pass on something I learned a few years back.

 

[86HO5.0]

Fixed it.

 

[keel]

Okay I got around to checking which kind of brake caliper I have. The outside of the pistons are smooth so its the 99-04 caliper.

Based on what you guys have suggested, I'll try a 93 Cobra booster and stick with the 93 Cobra MC I already have.

Any lesser known tips for getting the booster in? I would really rather not have to beat my strut tower in for clearance.

 

[gkomo]

Somewhere i saw someone say there is a plate inside the cabin where the bolts of the booster go through the firewall. If you remove that plate it allows you to stick the booster bolts through the firewall at a greater side angle (like away from the strut tower) to get it into position, whereas if that plate is left in there you have to go more straight forward. I've never done this mod so this is just me going off memory of something i heard somewhere sometime ago, so this information would need to be verified.

 

[nickyb]

Some beat the outta the strut tower,some elongate the holes,and some just get lucky and it goes right in.

 

[General karthief]

Fixed it.

The combo valve?