How to understand custom brakes

If you've seen THE EPIC GEMINI FINALE (CLICK HERE) Marty buttoned up a whole bunch of fiddly jobs to get the Gemini driving and engineered for road use, and a big one was finishing off his custom brakes. Now, because your ability to stop in an emergency rides on the brakes, any modifications to the braking system of your car needs special care. 

So, let's take a look at some considerations when setting up custom braking systems and the modifications you might have to make.

Your brakes are a closed hydraulic circuit, which will normally use vacuum assistance from the engine to make the pedal easier to press. As you press on the brake pedal it pushes a pushrod in the master cylinder to force brake fluid down the lines, which then pushes the pistons in brake calipers onto the discs (or pushes wheel cylinders out to push the brake shoes onto the brake drum housing). This is how your car stops. 

Upgrading the size of the calipers, switching from un-boosted (manual) to a vacuum-assisted set-up, or changing from drums to disc brakes will normally require many other changes so it is important to understand what might need to be changed before you go buying those race-spec calipers off Marketplace.

One of the jobs Marty had was to set up a custom brake proportioning valve (also known as a "bias adapter") on the car, as the upgrades done to the Gemini's braking system had meant the front:rear balance was thrown out. The bias needs to be right because if the system applies too much rear brake it means your car could go out of control in a panic stop situation. 

The engineer signing off on the Gemini wanted Marty to adjust the bias slightly, so this meant he had to change some brake lines, which is a common modification on builds like Marty's Gemini.

Brake fluid travels from the master cylinder through steel lines down to the calipers. We've all seen Marty and MOOG fit braided brake lines to their projects in the past, as the braided lines resist swelling compared to the stock rubber lines that stock cars come with (and this improves brake pedal feel). 

The steel lines are also called hardlines and you'll need special tools to whip up a custom set for your car. This will happen when changing master cylinder or proportioning valve, and when you're changing from drum brakes to discs on some cars. 

The hardlines attach to the rest of the brake system using special nuts (also known as "olives"), which clamp the system together and hold the pressure. These are crucial for safety, because if the system leaks or the nuts blow apart, you'll lose braking performance (and if you need someone to explain why that's bad, maybe stop reading and have a good hard think about how unsafe that is). 

The nuts go onto the hardline (threaded end facing you, always), then you bend it into shape and (once you're happy with the fit) you then flare the end. 

You can see in the image below the two metal brake lines have different looking bumps on their ends. These are flares which are designed to lock hard against the end of the fittings, and prevent fluid leaking out. Some cars will have double flares while others will have single flares, and pairing these properly is crucial to having your brakes work properly. 

When making new hardlines you'll have to flare the steel tube, and understanding what type of flare is required can literally be the difference between having brakes that work and brakes that leak and don't hold pressure. 

Of course, these hardlines have to bend around the engine bay so you'll need a way to make them snake through and around everything. Brake hardline is most commonly made with stainless steel tube of around 3/16in diametre, and it comes in a round loop you'll have to straighten out before making your lines.

This is where you use a tube-bender to create the correct-radius bends for your car. Using anything other than a proper bending tool will end up kinking the line, which then means the fluid won't flow through. 

Benders come in two flavours, the cheap (top image) or the primo (bottom image) which can also flare the end of the line. Just don't forget to fit the 

Obviously, it is always better to do this before you fit an engine to the car, or before you fill the system with brake fluid (as its HIGHLY corrosive) but that is rarely the case.

When upgrading the calipers to larger units, or when switching from drums to disc (either on the front, or all 'round), you will often have to upgrade your master cylinder to suit this new set-up. The bore of the master cylinder and depth of the pushrod can both come in different sizes, and there are many calculators online which can help working out what size master cylinder you need for your car. 

The final part of the braking system which can often need modifications when customising your brakes is the pedal itself. Changing from a brake system that doesn't use a booster to one with vacuum assistance will require modifying how your brake pedal pivots, so you can maintain good pedal feel. 

This is not something a novice should be doing, as it is illegal in many states of Australia to modify a brake pedal. But it is something to think about when investigating brake upgrades for your car, as having great brakes should be a goal for every car enthusiast - none of us want to find ourselves unable to stop in an emergency.