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Hydraulic System & Lines

Hydraulic System & Lines

Learn and Shop: Brake Hydraulics

Brake systems on modern vehicles are actuated by hydraulic power. Hydraulic systems work by transferring force though fluid from one end of a sealed system to another. This sort of remote power transfer is ideal for braking systems, where there is one input source (the brake pedal) and multiple output sources (each corner with a brake piston). The major components of hydraulic braking systems are the master cylinder, proportioning valve, brake lines (both hard, and flexible), individual cylinders at the brakes, various components related to Anti-lock Braking Systems (ABS), and the brake fluid itself.


There are different kinds of brake fluid, but they all serve the same function. Brake fluid is the medium through which power is transferred through the system. Important features of brake fluids are that they are non-compressible and they have a high boiling point. When fluids boil, they give off gas. Since gases are compressible, if brake fluid were to boil, fluid power would be lost and the brakes would cease to function when the pedal is pressed. Further brake fluid discussion can be found in the "Tool & Chemical" category.

Brake Lines

Brake lines that carry the fluid are custom-bent hard-lines that route from the master cylinder, to the the proportioning valve, and to each corner of the vehicle. Because the wheels move with the suspension and steering systems, a flexible portion of the line is needed from the hard lines to the brake pistons. Most OEMs will specify rubber hoses for those sections. While low in cost and suitable for daily use, rubber lines are subject to expansion which can dull pedal feel. They are also susceptible to rot and damage from sharp objects. For performance applications it is common to replace these rubber sections with Teflon/nylon lines with a stainless-steel braided sheathing. These lines are more durable, and provide improved pedal feel, braking power, and consistency.

Master Cylinders

The master cylinder translates power from the brake pedal to hydraulic power. It has a push-rod input, and multiple line-outs. The master cylinder may also house a reservoir to hold additional brake fluid to feed the hydraulic system. The cap of the reservoir commonly has a float with a sensor, to warn of low fluid levels. Common failure modes with master cylinders involve internal or external leaks. External leaks are rare, but relatively obvious. Internal leaks involve fluid bypassing the internal piston seal, causing reduced pedal pressure.

Brake Bias

Because the physics of braking cause a forward weight transfer of the vehicle, more braking is needed in the front wheels where they are more effective. Although the rear brakes are less powerful, they provide stability under braking. Fluid power is diverted in a specific front-to-rear bias though a steady-state device called a proportioning valve. Normally, this valve is not modified or changed, but in racing cars it is common to find adjustable proportioning valves, to bias braking front or rear as conditions demand.

Power Delivery

Brake calipers and wheel cylinders are the service devices for the hydraulic power. Pistons in these parts act on brake pad or shoe linings to slow the vehicle. As pressure is applied to the hydraulic fluid, pressure is imparted on the cylinders at each wheel in a proportional amount. It is fairly direct system, apart from the added pressure given from the brake booster.

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