Several things happen when you stand on the brakes hard. Foremost, the system takes the energy of momentum and transduces it into heat, and it slows the car. That’s obvious.
What’s not so obvious are other things that happen. For example, under hard braking the firewall flexes and the entire brake booster and master cylinder push forward in the engine compartment. If you’re using regular old brake fluid, it can boil and introduce air into the system. In addition, the flexible rubber brake hoses that extend from the chassis to the calipers also expand with increased pedal pressure, taking the clamping energy away from the caliper piston.
In Spec Miata, you’re not allowed to brace the brake booster and master cylinder, but you are allowed to replace the brake hoses with stainless-steel braided hoses, which transmit more fluid pressure to the caliper piston, adding stopping power and giving the driver better pedal feel.
These brake hoses should be one of the first things you replace if you plan to put a Miata on track, whether it’s for racing or for HPDE. The installation isn’t hard, but it is messy. Have lots of brake cleaner spray handy, and be sure the fittings are clean upon reassembly. Dirt in the fittings can make them feel tight before they’re actually tight, and that can lead to leaks when under pressure.
With proper hand tools — good line wrenches are a must — the installation shouldn’t take more than two hours and a nominal amount of profanity. Here’s how the job plays out.
You’re going to make a big mess of your garage floor, but clamping off the brake hoses keeps some fluid from dribbling all over the place when you remove the bolt/fitting that holds the brake hose on the caliper. Have lots of newspaper under each caliper to absorb the old brake fluid.
On a Miata, the right rear hose includes a junction block that connects to the hard line that leads to the left rear caliper. It’s important to use line wrenches, not box wrenches, so you don’t round-off the brake line fittings, which can be stubborn.
You’re going to have to bleed the whole system with fresh fluid when you’re done, so it doesn’t matter which caliper you begin with. When bleeding the system on a Miata, always start with the left rear caliper because it’s farthest from the master cylinder.
Use a line wrench to loosen the hard line on top of the junction block, which leads to the left rear caliper.
The factory junction block is held on with two 10 mm nuts. Remove them and the assembly comes right off.
Any number of vendors sell stainless-steel brake hoses for Miatas. We chose these from 949 Racing. The aftermarket junction block only requires one mounting nut. Tighten the nut and brake line fittings to the junction block and connect the hose to the caliper. The copper washers go on both sides of the banjo fitting. The opening photo shows the proper stack.
Moving on to the left rear caliper, clamp the brake hose and remove the line nut behind the mounting bracket. Again, line wrenches are must.
The flexible line is held in place with a spring clip behind the bracket. The aftermarket lines also use these clips so don’t lose them.
Use a line wrench on the brake line fitting and a regular wrench on the brake hose. Again, spray brake cleaner on the fittings to keep the threads clean and free of debris.
When you tighten the rear fittings at the calipers, you have to hold the fitting in place so it sits down on the caliper. You also want to tighten the fittings so the hoses don’t bind as the suspension travels up and down.
By the time I got to the front brakes, I had made such a mess I didn’t bother clamping the hoses. On the front calipers, remove the fitting that holds the brake hose to the caliper.
Use the line wrench to remove the brake line nut from the inner fitting on the brake hose.
A screwdriver or needle-nose pliers are ideal for removing the spring clips that hold the brake hose into the chassis bracket. These can fly out and across the garage, but you do need them to hold the aftermarket hoses, too.
Just like you did on the rear, use the line wrench on the brake line fitting and the regular wrench on the brake hose fitting. Why? The brake hose has a larger hex fitting and it’s much less likely to round-off like the smaller line fitting on the chassis.
Route the brake hose so it doesn’t bind with suspension travel or steering. Notice how there’s a copper washer on either side of the banjo fitting where the hose attaches to the caliper. They’re essential to leak-free operation. Now bleed the whole system with some good high-temperature racing brake fluid.