C4 ZR-1 Upgrades - The Purple Project, Part 2

C4 Adventures: On The Road And Under The Hood

Hib Halverson Apr 1, 2000 0 Comment(s)
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The best thing to do, if you want to reduce the potential for brake fade due to out-gassing, is have them grooved. This procedure requires a machine shop experienced in brake rotor modification and a NC or CNC mill. Global West has both.

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Back at the shop, I installed the brake hardware. With a couple of exceptions, I followed the instructions in the Service Manual. The book calls for new caliper mounting bracket bolts each time one removes the calipers, but I see that as an unnecessary expense as long as the bolts and their bolt holes are in good condition. I carefully cleaned the bolt threads and the threads in each front and rear knuckle, applied Valco Cincinnati Thread Locker to the bolts, and installed the parts.

Caution! The 1995 and 1996 Service Manuals incorrectly list the rear caliper mounting bracket bolt torque. For all '88-96 rear brakes, the correct figure is 70 ft-lbs. When it is misprinted, the front bolt torque is listed for the rear bolts. If the rear bolts have been tightened to that incorrect value, damage to the rear knuckles will occur. When changing the rear brakes on a '95 or '96, carefully inspect the caliper mounting bolts and the bolt holes in the knuckle. If the bolt hole threads are damaged due to overtorque, the knuckle must either be replaced or have its threads repaired with a TimeSert kit, which is discussed later in this article. Also, in that case, new rear caliper bracket bolts are required.

I decided that until I actually take our purple project car to a race track, I'd use the OE brake pads, so on went all four "loaded" calipers right out of the GM boxes. Further improvement in pedal feel comes if the production brake hoses are replaced with braided, stainless steel covered, Teflon brake hoses. Doug Rippie markets braided brake hoses for C4s, so I installed a set (PN10-105).

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Next, I flushed the system. In the case of a '95 ZR-1, a pressure bleeder must be used because the ABS controller's brake fluid prime pipe is not pressurized when the brake pedal is stroked during a "manual" bleeding session. I use a Mac Tools bleeder (PN BBT2), but there are a wide variety of similar devices on the market.

There are three different ABS brake pressure modulators used on '86-96 C4s. All are located in the well behind the driver seat. The '86-91 brake pressure modulator is not easily bled. The '92-94 base and '92-95 ZR-1 BPM is bled via a fitting on its side. When doing this, take care to avoid spilling brake fluid in the well. The '94-96 non-ZR-1 "Electronic Brake and Traction Control Module" (EBTCM) can only be bled using a scan tester, such as the Vetronix TECH1A or Mastertech, both of which activate the unit's "autobleed" function.

The rest of the bleeding procedure is pretty conventional and can be done with a pressure bleeder or manually. Interestingly, I use Ford brake fluid (PN C6AZ-19542-AB). Back in the early '90s, I was an observer at a Corvette development test session at a race track near Grattan, Michigan. As I watched the development engineers work on the cars, I noticed a cache of Ford brake fluid in their tool chest. Intrigued with its presence, I inquired. I was told that Corvette development had tested a number of brake fluids, including GM's own brand, and found the Ford to be best by a slight margin. When doing racetrack testing, which taxes brake fluid to the maximum, they wanted the safety margin provided by the Ford fluid.

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The other reasons I like Ford fluid for street high-performance applications are: 1) value-the product performs well and has a reasonable cost, and 2) a low-level of hygroscopicity (moisture absorption) typical of factory-fill brake fluids that works well in an application where the frequent fluid changes associated with race cars are not an option.

Whatever fluid you pick, do not use silicone-based brake fluid in any '86 or later Corvette. Silicone fluids are not compatible with the antilock braking systems (ABS) on those cars.

The last step in our brake thrash was installation of a Mid America Designs front air dam with integral brake cooling ducts (PN 18721). This replaces the stock air dam and accepts ducts that run to the inside of the brake discs and direct cooling air at each disc's cooling vents. It takes several hours to install using common tools. We did run into one problem-the cable ties that come with the kit. They are a weak design, and we broke all but two trying to tighten them as we tied down the cooling ducts to suspension parts. We needed a more robust tie, like the type intended for electrical work, and we found a "variety pack" of cable ties at Costco Wholesale. When installing the kit, put the ducts in place first, starting from the outlet next to the inside of each brake rotor. Work forward, attaching the cooling duct as shown in the photo. Put the Mid America air dam outboard sections in last and connect the ducts to them. While Mid America supplies an entire air damn, we saved the new centersection for when our OE center finally wears out.


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