DIY Auto Service: How Hydraulic Brake Systems Work

Updated on January 16, 2019
Michael M Thomas profile image

Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."

Principles of Hydraulic Brakes

Brake systems need to be very powerful because they need to stop a heavy vehicle going a given speed in a short distance. The momentum of the vehicle is kinetic energy. Since energy cannot be destroyed, only changed, the brake system transforms this momentum into heat energy.

The brake system must be able to dissipate this heat quickly. If the brake system is overheated, it could cause the brakes to “fade”. Brake fade happens when the brakes can no longer turn the momentum into heat. The harder the brake pedal is pushed, the less the brakes work. On highways with steep downhill grades, it is common to see a “Truck Runaway Ramp” that can be used by a truck that has lost its brakes.

The coefficient of friction is the amount of friction between two objects. The coefficient of friction between the tires and dry concrete is going to be much higher than between tires and an ice-covered road. The coefficient of friction for the brake pads or shoes to the rotor or drums can also affect the brake performance. If the brake shoes are contaminated with grease due to a leaking wheel seal, the shoes' coefficient of friction will be lower, and the brake won’t perform as desired.

Pascal's Laws of Hydraulics

The force created is equal to the Hydraulic pressure pushing on the square inches of the piston. F=AxP
The force created is equal to the Hydraulic pressure pushing on the square inches of the piston. F=AxP

Pascal's Laws of hydraulics

  1. Liquids are not compressible.

  2. Liquids will take the shape of their container.

  3. The area of the piston multiplied by the pressure in PSI (Pounds per Square Inch) will equal the force generated by the piston, or A X P = F.
  4. Pressure applied to a confined liquid will exert equal pressure in all directions.


  • 100 PSI X 10-square-inch piston = 1,000 lbs of force.
  • 1,000 PSI X 10-square-inch piston = 10,000 lbs of force.

Hydraulic Brake System Pressures

In the hydraulic brake system, the pressure is created by the driver’s foot. The formula would be altered to read:

  • Mechanical Force ÷ Area = Pressure or F ÷ A = P

  • Example: 100 lbs. of force ÷ 1 Square inch Piston = 100 PSI in the brake system.

Now that the pressure has been created in the brake system, the pressure will react on the pistons in the disc brakes and drum brakes. Using Pascal’s laws, the pistons will multiply the pressure by the area of the pistons. Using the example from above, a disc brake piston of 6 square inches would apply a force on the disc brake pads.

  • Pressure X Area = Force

  • 100 PSI X 6” = 600 lbs of Force

Brake Fluids

Brake fluid is a non-compressible liquid. If brake fluid is contaminated with air in the system, the air is compressible. This would cause the brake pedal to feel mushy or spongy and not solid. Most brake fluids attract moisture and are called hygroscopic. The moisture in the brake fluid will boil at a lower temperature and turn into a gas which is compressible. The moisture could also cause corrosion in the brake system.

Brake Fluid Types

Brake fluid must meet specific criteria and is rated by the DOT. These criteria involve the boiling point, freezing point and the reaction to rubber parts in the brake system.

The following are the brake fluid types used today:

  • DOT 3 – boiling point = 401°F, Wet Boiling Point 284°F, actively hygroscopic.

  • DOT 4 – boiling point = 446°F, Wet Boiling Point 311°F, actively hygroscopic.

  • DOT 5.1 – boiling point = 500°F, Wet Boiling Point 356°F, actively hygroscopic.

  • DOT 5 – boiling point = 500°F, Wet Boiling Point 356°F, non-hygroscopic.

Brake Fluid Recommendations

Most manufacturers recommend DOT 3 brake fluid in their brake systems. Although in theory DOT 3, DOT 4 and DOT 5.1 can be mixed together, most manufacturers do not recommend it. Most manufacturers’ recommendation is right on the Master Cylinder cover. For the best results, stick with the recommendation and don’t mix fluids.

DOT 5 is a silicone based brake fluid that does not attract moisture, but should NEVER be mixed with any of the other brake fluids. Most manufacturers DO NOT recommend the use of DOT 5 brake fluid.

Never get brake fluid on painted surfaces as the brake fluid will eat into the paint.

Most manufacturers recommend flushing the old brake fluid about every 2 years. Check their recommendations.

Never mix any mineral oils into the brake system because they will cause the seals to swell and fail. Every seal in the system will have to be replaced if engine, power steering or transmission oil has been added to the brake system.

Do not leave the can or the brake system open and exposed to the air as it will absorb moisture out of the air.

Basic Hydraulic Brake System Components

The hydraulic brake system starts at the two-part master cylinder, a hydraulic pump, operated by the driver’s foot. High-pressure steel lines carry the brake fluid to control valving and finally connect thru flexible rubber lines to the disc brake caliper or the drum brake wheel cylinders.

Rigid Lines

The hydraulic brake system uses “High-Pressure Double Walled Seamless” brake lines. The lines are also treated to resist rust. These lines connect the master cylinder to the control valves. The control valves to the ABS Unit and the ABS Unit to the frame close to the axles. The end of the brake lines are typically clipped to the frame or body. Flexible lines are used from the body to the wheels or drive axle.

Coming off the master cylinder the lines are typically coiled to cushion the movement between the body and frame. Rigid lines could fatigue and crack.

Bending the rigid lines should be done with a tubing bender so the line doesn’t kink. A kink in the line would cause a restriction and could also weaken the line.

The ends of the lines are flared with either a “Double Flare” or a “Bubble or Ball Flare”. When making up a brake line always use “Brake Line” and no other substitute. Brake line can be purchased in a variety of lengths. For long lengths, a union can be used to connect the two shorter lines. If the right length line cannot be found, a longer line can be cut with a tubing cutter and the end flared to the proper flare type.

Flexible Lines

Flexible lines are used at the wheels and rear axle. The rigid lines would kink, crack and break due to the movement. The flexible lines are a multilayer construction. The outer cover is made to protect the inner line and give it support. The outer shell takes the most abuse. When it looks cracked or dry-rotted it is time to replace the line. The line typically connects to the disc brake caliper with a hollow bolt and is sealed by a copper crush washer. At the wheel cylinders of drum brakes, the line screws directly into the wheel cylinder. The flexible line is replaced as an assembly and not repairable.

Flexible lines can be easily damaged by hanging the brake caliper by the line or clamping the line off with vice grip pliers. The damage may not be visible because the inside of the line is where the damage occurred.

Dual Master Cylinder

The Dual Master Cylinder has two sections to divide the brake system if one part fails.
The Dual Master Cylinder has two sections to divide the brake system if one part fails.

Master Cylinder

The master cylinder is actually two circuits in one. Required by the DOT, the master cylinder is a Dual Master Cylinder. This means the hydraulic system is divided into two parts. This division allows for part of the brake system to function if there is a failure in the other part. Years ago, vehicles had a single master cylinder and if there was a hydraulic failure anywhere in the system, all hydraulic brakes were lost.

Parts of the Master Cylinder

The master cylinder is made up of several parts that work together.

  • The Reservoir stores the brake fluid. Divided into two sections to divide the two hydraulic systems. The reservoir could be part of the master cylinder housing or more popular today, an attached plastic reservoir. The plastic reservoirs can be translucent to allow the fluid to be checked without opening the reservoir.

  • The Reservoir Cover or covers have a rubber gasket that will expand as the fluid level goes down. This will keep air and moisture separated from the fluid. If this gasket is damaged or not sealing the brake fluid will absorb moisture from the air. If this gasket is deformed, it may be an indication of contamination in the fluid.

  • The Master Cylinder Housing is made from cast iron or aluminum. The cast iron master cylinders have the ability to be rebuilt. The aluminum master cylinders should be replaced.

  • The Primary Piston is the piston in the rear of the master cylinder. To prevent fluid leaking out the back of the master cylinder, a bore seal is used that may be referred to as a secondary seal. Towards the front of the primary piston is a forward facing lip seal that is a pressure producing Primary Seal. On the front of the primary piston is a short rod and spring. The short rod will apply the secondary piston if the primary piston seals fail.

  • The Secondary Piston is the piston towards the front of the master cylinder. On the rear of the secondary piston is a back facing lip seal called a Secondary Seal. The secondary seal is used to catch the pressure from the Primary Piston Seal. This forms a hydraulic link between the Primary Piston and the Secondary Piston.

  • The Secondary Piston Primary Seal is a forward facing seal that creates the pressure for the second hydraulic system.

  • Two holes, a Replenishing (Inlet) Port and Compensating (Vent) Port are drilled between the reservoir and the primary piston and at the secondary piston. When the pistons are in their beginning position, the primary seal lip of each piston is sitting between these ports. See figure 8.

  • Two Hydraulic Outlets connect to the brake system. Each outlet connects to half of the brake system. This is the division of the hydraulic system that protects the vehicle from having no brakes. Though the vehicle won’t have full braking, some brakes are better than no brakes as would have been the case in a single master cylinder vehicle.

Hydraulic Disc Brake Operation

Hydraulic Disc Brake Operation with the pedal released and applied causing the brake pads against the rotor.
Hydraulic Disc Brake Operation with the pedal released and applied causing the brake pads against the rotor. | Source

Master Cylinder Operation

The Master Cylinder starts working as the driver presses down on the brake pedal. A rod connected to the brake pedal is seated in the Primary (rear) Piston. The rod moves the Primary Piston forward. This covers the Vent Port and traps the brake fluid in the bore. As the Primary Piston moves forward, it pushes the fluid towards the Secondary Piston. The rearward facing Secondary Seal traps the fluid and starts to move the Secondary Piston with a hydraulic link. The Secondary Piston covers the Vent Port and traps the secondary fluid. The fluid moves out into both outlet lines and the brake system. This trapped fluid reacts on the Disc Brake Caliper Pistons and/or the Drum Brake Wheel Cylinder Pistons.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.


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