Hydraulic systems are used to actuate the brakes of all trucks covered by this guide, as well as most vehicles on the roads today. The system transports the power required to force frictional surfaces of the braking system together from the pedal to the individual brake units at each wheel. A hydraulic system is used for two reasons. First, fluid under pressure can be carried to all parts of an automobile by small pipes and flexible hoses without taking up a significant amount of room or posing routing problems. Second, a great mechanical advantage can be given to the brake pedal end of the system, and the foot pressure required to actuate the brakes can be reduced by making the surface area of the master cylinder pistons smaller than that of any of the pistons in the wheel cylinders or calipers.
The hydraulic systems used by these trucks usually consists of a power booster, a master cylinder, a combination valve, front disc and rear drum assemblies, along with the pipes and lines that connect them. Additional components are added to vehicles equipped with an Anti-Lock Brake (ABS) system. For more information on the ABS systems utilized by these trucks, please refer to the coverage later in this section.
The master cylinder, mounted on the left-firewall or power booster, consists of two fluid reservoirs, a primary (rear) cylinder, a secondary (front) cylinder and springs. The reservoirs, although independent of one another, are contained within the same housing; fluid cannot pass from one to the other. Usually, the rear reservoir supplies fluid to the front brakes while the front reservoir supplies fluid to the rear brakes. By hydraulically separating the front and rear braking systems the vehicle is protected from most single point fluid leaks. In most cases should a brake line, caliper or wheel cylinder leak and the fluid level drop in that circuit (causing those brake components to stop working properly), the other circuit should still function providing reduced breaking ability, but preventing a total brake failure.
The dual circuit master cylinders utilized by these trucks employ two pistons, located one behind the other, in the same cylinder. The primary piston is actuated directly by mechanical linkage from the brake pedal through the power booster. The secondary piston is actuated by fluid trapped between the two pistons. If a leak develops in front of the secondary piston, it moves forward until it bottoms against the front of the master cylinder, and the fluid trapped between the pistons will operate the front brakes. If the rear brakes (or brakes which are actuated by the primary circuit) develop a leak, the primary piston will move forward until direct contact with the secondary piston takes place, and it will force the secondary piston to actuate the rear brakes. In either case, the brake pedal moves farther when the brakes are applied, and less braking power is available.
The hydraulic system operates as follows: When at rest, the entire system, from the pistons in the master cylinder to the pistons in the wheel cylinders and calipers, is full of brake fluid. Upon application of the brake pedal, fluid trapped in front of the master cylinder piston(s) is forced through the lines towards the wheels. On the way to the wheels the fluid will first pass through the combination valve (which is usually mounted to a bracket just below the master cylinder). Here it is monitored and proportionally distributed to the front or rear brake systems. Should a loss of pressure occur in one system, the other system will provide enough braking pressure to stop the vehicle. Also, should a loss of pressure in one system occur, the differential warning switch (usually located on the combination valve) will turn ON the brake warning light (located on the dash board).
Steel lines carry the brake fluid to a point on the vehicle's frame near each of the wheels. The fluid is then carried to the calipers or wheel cylinders by flexible tubes in order to allow for suspension and steering movements. As the fluid enters each brake caliper or wheel cylinder, the pistons are forced outward. The outward movement of the pistons force the brake pads against a round flat disc or the brake shoes against a round metal drum. The brake lining attached to the pads or shoes comes in contact with the revolving disc or drum, causing friction, which brings the wheel to a stop.
All pistons employ some type of seal, usually made of rubber, to prevent fluid leakage. A rubber dust boot seals the outer end of the cylinder against dust and dirt. The boot fits around the outer end of the piston on disc brake calipers, and around the brake actuating rod on wheel cylinders.
In time, the brake linings wear down. If not replaced, their metal support plates (bonded type) or rivet heads (riveted type) will come in contact with the disc or drum; damage to the disc or drum will occur and reduced breaking will be experienced. As a general rule, never use brake pads or shoes with a lining thickness less than 1 / 32 in. (0.8mm), though specifications and local laws should be checked for more stringent requirements.
Most manufacturers provide a wear sensor, a piece of spring steel attached to the rear edge of the inner brake pad (disc brakes). When the pad wears to the replacement thickness, the sensor will contact the disc and produce a high pitched squeal.
The hydraulic system may be checked for leaks by applying pressure to the pedal gradually and steadily. If the pedal sinks very slowly to the floor, the system has a leak. This is not to be confused with a springy or spongy feel due to the compression of air within the lines. If the system leaks, there will be a gradual change in the position of the pedal with a constant pressure.
Check for leaks along all lines and at wheel cylinders. If no external leaks are found, the problem is likely inside the master cylinder.