Fleets operating Class 1 through 5, or select Class 6 vehicles, utilize hydraulic braking systems. As with all vehicle systems, proper maintenance is crucial in ensuring vehicle uptime and safety. A well-maintained braking system is of paramount importance in establishing an environment for both fleet vehicles and public motorists to safely maneuver the roadways. Understanding the basic mechanics of the hydraulic braking system unveils the critical components that fleet technicians need to monitor and service in order to keep the vehicle in optimal working condition.

Hydraulic brake basics

The hydraulic braking system operates on a fairly simple concept of force. A driver applies force to the brake pedal, and that force is transferred to the wheel-ends where the brake friction material contacts the rotor or drum, reducing the kinetic energy and, in turn, reducing the vehicle’s speed.

“A hydraulic brake system uses a thick, [virtually] incompressible fluid to transmit the driver’s pedal force to the wheel-ends,” says Tom Jendrusch, product design engineer, AERO Light Repair Engineering, of American multinational automaker Ford Motor Company. “The pedal force is converted to a hydraulic pressure in the master cylinder. With a disc brake, this pressure acts as a force against the caliper piston, pressing the piston out so that the brake pads attached to the caliper press against the rotating rotor and generate friction, or heat. With a drum brake, the pressure forces the brake shoes against the drum to create the friction. The rotor, or drum, which has the wheel and tire attached, slows down causing the vehicle to slow down.”

Furthermore, the applied pedal force is proportional to the braking force, thereby giving the driver the ability to control the rate of vehicle deceleration, Jendrusch says.

Hydraulic braking systems are specified on Class 1 through 5 vehicles with a Gross Vehicle Weight Rating (GVWR) of up to 19,500 lbs. Class 7 and 8 vehicles, with a GVWR of more than 26,000 lbs, are specified with air braking systems. Class 6 vehicles, within the 19,500- to 26,000-lb range, may be specified with either hydraulic or air braking systems. However, hydraulic braking systems are never specified on vehicles with a GVWR greater than 26,000 lbs, as hydraulic brakes have limitations to the amount of brake force that can be generated, Jendrusch says.

“Cost, packaging, and ease of operation are the most obvious benefits [of hydraulic brakes],” Jendrusch continues.

Another benefit is that operating a medium duty truck with a GVWR of no more than 26,000 lbs does not require a Commercial Driver’s License (CDL) if the vehicle is specified with hydraulic brakes, he adds.

Maintenance considerations

There are certain aspects of the hydraulic braking system that technicians should be mindful of; such attentiveness will allow them to spot warning signs of an impending failure and can ultimately prevent unwanted downtime.

Jendrusch recommends technicians check that nothing is touching or rubbing against the brake tubes and hoses, as such contact could lead to premature wear and failure of these components. Unwanted contact could lead to wearing of corrosion protection which could cause tubing deterioration. Jendrusch further recommends that caliper guide pins are properly lubricated. Proper lubrication will ensure that the caliper operates and slides as designed, promoting even brake pad wear.

“Whenever the calipers are serviced, the lubrication of the guide pins should be checked to ensure free movement of the caliper,” Jendrusch emphasizes.

When service technicians inspect vehicles coming into the shop, they should always check the braking system, and specifically the brake pads, advises Jeremiah Terry, general manager of Phoenix Systems, a provider of tools and technology for the auto care industry. “They might want to watch for any sort of wobbling or shaking that might [indicate] a rotor that’s warped … They want to look for any squeals or loud sounds coming from the brake system,” Terry suggests. “And then, last of all, specific to hydraulic is, of course, the brake fluid.”

Brake fluid is the lifeblood of the hydraulic braking system. Its performance and status affects the hydraulic braking system in its entirety, so proper maintenance is of the utmost importance.

One visible warning sign drivers and technicians alike should watch for is the “low brake fluid” indicator light on the vehicle dashboard, typically represented as an exclamation point within a parenthesized circle.

“Brake fluid wears out over time, but not from just sitting,” Terry says. “It takes driving. The brakes get really hot when driving down a hill, from stop-and-go driving, [et cetera]. That heat is transferred into the brake fluid; the heat, plus oxygen, seeps in through the rubber hoses as well as through the cap and the master cylinder.”

The combination of heat and oxygen in the brake fluid begins to break down the fluid and leads to corrosion inside of the braking system, Terry warns. The first victim of the corrosion process is the copper inside of the brake lining. As the amount of copper increases in the brake fluid, it starts “attacking” all the iron components in the braking system.

“That is when people usually will get stuck calipers,” he says. “They’ll come in and their caliper is stuck, it’s pinched tight, or maybe the caliper is not coming out. And so, the [vehicle] is pulling to the right or the left. This is all caused by corrosion taking place inside the brake system because the brake fluid is worn out.”

To avoid such corrosion, Terry recommends utilizing Phoenix Systems’ BrakeStrip or the BrakeStrip Plus. The BrakeStrip is a test strip for copper in the brake fluid; the strip can be dipped in the brake fluid to determine the level of copper within the fluid. Doing so reveals how much corrosion is occurring in the braking system. Terry notes that a vehicle’s brake fluid should be changed before the fluid measures at 200 part per million of copper.

Terry further recommends that fleets test the copper levels of the brake fluid each time the vehicle is in for an oil change.

“In a fleet, if the vehicles are all being used for the same type of operation, same type of driving, then they’ll start seeing a mileage correlation [as to] when the brake fluid fails,” he says. “That being said, though, that [assumes] they all started with the same brand of brake fluid, [as] every brand is different.”

Brake bleeds

Technicians will need to bleed the brakes whenever air has entered the hydraulic braking system.

“Bleeding the brakes is required anytime the closed hydraulic system is opened,” Ford’s Jendrusch says. “Connections between the master cylinder, hydraulic control unit, calipers, wheel cylinders, and brake tubes and hoses are all sources of air getting into the system.”

Changing brake fluid, if done properly, should not introduce air into the system, says Phoenix Systems’ Terry. The most common instance of air entering the system is when a part or component of the system is replaced, he continues, upon which the brakes may need to be bled.

“Excessive pedal travel and low braking force, [sometimes referred to as a] soft pedal, usually indicate that air has entered the closed hydraulic system,” Jendrusch says. “Air will compress, which quickly uses the pedal travel and reduces the amount of force and hydraulic pressure that can be applied to the wheel-ends. The presence of air in the hydraulic system is observed in relatively long pedal travel and low, or no, braking force generated.”

The objective of a brake bleed is to force the air out of the hydraulic system. There are many different methods, procedures, and tools and equipment that may be utilized in order to complete a brake bleed.

The most tried and true method of a brake bleed is a two-person job. One person will apply the brake pedal as the other will open and close the bleed screws. Jendrusch outlines the process as follows:

1. Attach tubing (preferably clear so that any air bubbles can be seen) to the bleed screw. Have a brake fluid collection container available for the discharged fluid.
2. Pump the brake pedal a few times and then apply constant pressure.
3. Open the bleed screw to allow brake fluid/air bubbles to escape. Keep applying brake pedal force as the pedal sinks to the floor.
4. Tighten the bleed screw. Release the brake pedal. Replenish the fluid in the master cylinder so that it does not deplete all the fluid in the reservoir and allow more air into the system.
5. Repeat steps 2-4 as needed until the pedal feels firm.
6. Repeat steps 1-5 for the remaining three wheel-ends.

There are other tools and methods available as well to complete a brake bleed. There are pressurized bleeding tools, such as pneumatic or hand-pumped tools, that can be attached to the master cylinder reservoir to push the fluid from the top down. There are also vacuum pumps available that pull the fluid top down from the bleed screw.

“There are some drawbacks to both of those,” Terry advises. “With the vacuum, a lot of times they pull air around the bleed screw, so you’re never quite sure if you have the air bubbles out. And it takes a long time pulling the fluid through. Pressure methods are pretty good because you can push a lot of brake fluid through from the top down and its pretty quick. The only drawback to pressure is that sometimes the air doesn’t want to go down, it wants to go up.”

It was after taking these drawbacks into consideration that Phoenix Systems developed their reverse brake bleeders, the V12 and the Max Pro HD.

“Instead of vacuuming the fluid down or pushing it from the top down, we actually push it into the bleed screw and it pushes the fluid from the bottom up,” Terry explains of the operation of the reverse brake bleeders. “You empty the master cylinder and then you fill it back up by pushing the fluid into the bleed screw; that allows the air bubbles to go up the way they naturally want to go. And it ensures that you can have a pedal that has no sponginess that can stop the [vehicle] the way it was meant to be stopped.”

Phoenix Systems’ Terry recommends that technicians reference the vehicle owner’s manual before performing a brake bleed, as different vehicles may require different procedures and sequences.

“In general, it’s usually the furthest wheel from the master cylinder [that you start with],” he says.

Terry further advises technicians to be aware if the hydraulic braking system in service features a proportioning valve. As proportioning valves keep the fluid flowing in one direction, from top to bottom, they are usually restricted to a certain psi rating. To overcome the directional restriction, the psi of the brake fluid must surpass that of the proportioning valve. Furthermore, when bleeding brakes on newer vehicles, it is recommended technicians utilize a scan tool. Scan tools can be used to actuate ABS sensors and other valves. Vehicles with emergency braking systems need to be properly bled to insure the emergency braking system functions at the time of the emergency braking system’s initiation.

The hydraulic braking system is only as efficient as its level of maintenance. Monitoring brake fluid and system corrosion, maintaining adequate brake pressure, proper lubrication, and other best practices applied throughout the system will promote efficient system operation and, in the end, prolong the life of the system.