Air brakes get more attention, but hydraulic brakes are installed on more vehicles. Understanding how they work is the first step to safe, cost-effective diagnosis and repair.
Ever wonder why there can't be just one kind of brake? It's because air and hydraulic brakes each have operating characteristics that make one or the other ideal for certain applications.
In heavy-duty combination vehicles, air is the clear choice because of the large volume of liquid that would be needed to acadia all the wheel cylinders. Plus, dealing with gladhands and hoses filled with hydraulic fluid would be messy.
But for light and medium-duty straight-truck applications, hydraulic brakes offer advantages including:
Brake feel — that is, as the pedal is pressed farther down, effort increases;
High line pressures, which permit the use of lighter, more compact braking components;
Less initial expense, due to smaller and fewer components;
Cleanliness — hydraulic brakes are closed systems;
Ease of locating leaks, since fluid is visible.
There are many more permutations of hydraulic brake systems than found in air systems, but all have basic similarities.
THE HYDRAULIC SYSTEM
All hydraulic brake systems contain a fluid reservoir, a master cylinder, which produces hydraulic pressure, hydraulic lines and hoses to carry pressurized fluid to the brakes, and one or more wheel cylinder(s) on each wheel.
The wheel cylinders expand under fluid pressure, and force the brake shoes against the insides of the drums. If disc brakes are used, calipers, with integral cylinders, clamp down on the rotors when pressure is applied.
Because a vehicle must be able to stop much more quickly than it can accelerate, a tremendous amount of braking force is needed. Therefore, the retarding horsepower generated by the brakes must be several times that of the engine.
In order to develop the forces required to hold the brake linings against the drums or discs, and to achieve controlled deceleration, it is necessary to multiply the original force applied at the brake pedal....
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