The original brakes, built as per the instructions in the FFR manuals, were so terrible as to be dangerous. The pedal went halfway to the floor before you got any braking at all, and from there on the pedal was spongy and the brakes had little stopping power.
This turned out to be due to using the donor Mustang's power brake master cylinder, which is designed for drum rear brakes and has a smaller piston for the rear brakes than for the front. This master cylinder can't deliver adequate volume to the disc brakes used by the FFR IRS design.
To get reasonable brakes we ditched the Mustang power brake master cylinder (which was installed without booster per FFR instructions) and distribution block. We substituted a manual m/c from an earlier Mustang and used a simple tee instead of the Mustang distribution block, which isn't designed for a rear disc brake application.
The brake pedal was originally very long (about 3" travel before anything happened) and very spongy, and this got worse at the track. Also, due to the mismatch of distribution block and rear discs, the rears had too much braking and the car was unstable under hard braking. With the new setup, the pedal is very high and firm.
This is another thing I knew about in advance, due to reading the Cobra Forum and also from an experienced Cobra builder who emailed me, but again we were not sure. I thought the "stock" FFR setup should at least be adequate, and maybe it is with the stock drum brakes that come on the Mustang live axle. But we went with FFR's IRS option, which uses T-bird rear spindles and disc brakes. The Mustang power master cylinder plus these discs results in a configuration that I feel is actually dangerous.
We substituted a manual master cylinder from an '81 Mustang. Unlike the power master cylinder from the donor Mustang, this master has front and rear bores of equal diameter (7/8"). This provides enough fluid volume to the rear circuit to properly operate the rear discs.
This master cylinder apparently came in two versions, an aluminum version and a cast iron one. I was able to locate an aluminum one through the Carquest in Enfield, NH. The part number for the aluminum version is 20-1763; the iron one (available at other car parts stores) is 10-1763. I believe part number 10-1764 has slightly smaller bores, which would give slightly less pedal effort at the expense of longer travel.
Although the master cylinder swap helped a lot on the street, we still found that the pedal got "long" and soft as we put in laps at Loudon at the COMSCC event in October 2000. Bleeding the brakes helped, but they still weren't as good as we'd like.
We changed to a set of Z-rated brake pads at the front and installed a set of braided stainless steel/teflon front brake lines from Maximum Motorsports (part number MMBHKF).
The COMSCC staff stressed the importance of using braided stainless lines when upgrading the brake pads because of the extra heat generated by the harder pads. It's also important not to use full racing pads for time trials and autocrosses because they are too hard and will never get hot enough to work well in the short runs we do in these type of events. The Z-rated pads are the best compromise for this type of activity, and work well on the street too.
We found that our Cobra's brakes felt and worked superbly at Lime Rock with this setup. The pedal was very high and quite firm, as a race car's should be, and it didn't go down or get softer as we put on laps.
However, it was quite cool that day and Lime Rock has only one significant braking area. It will be interesting to see how they hold up on a hot day at Loudon, which is probably a lot harder on brakes.
Update 2001: The Z-rated pads did not cure the braking problems at tracks like Loudon, where the brakes are used heavily and frequently. Eventually we made these changes:
With these changes, the brakes were much more consistent, and didn't require bleeding at the end of every track day. However, front pad wear was still high, and the el cheapo rotors we were getting from Auto Zone didn't hold up very well either.
Update 2002: For 2002, we acquired a set of PBR calipers and 13" rotors for the front. These are the same calipers that are used on early model Mustang Cobra R's and Cobras. These required going to SN95 spindles. We used early SN95 spindles, from '94, because from '96 on these spindles have a significant increase in track, which would have required us to use wheels with greater offset.
The street pads that came with the PBR kit turned out to be pretty much useless on the track, so we switched to Porterfield R4's.
The fancy-looking gold-plated and drilled rotors that came with the kit also turned out to be useless on the track; we destroyed them within two events. They developed serious cracks, some of which went all the way from the outer holes right through to the outer edge of the rotors!
We replaced these with a set of slotted TRW rotors, which are still going strong at the end of 2003.