Auto Parts Different Systems of Handbrake Operation
‘Fail safe’ Auto braking systems
Tandem Dual Master Cylinder
The weakness of any hydraulic circuit is that leakage at any point puts the whole system out of action.
A dual-braking system, which has two independent hydraulic circuits, overcomes this problem. The earlier dual- braking systems had a master cylinder with one piston to operate the front brakes only and another for the rear brakes. If one circuit failed, the car still had brakes.
In the more complex split system (below, right), each front disc brake has a pair of pads with individual cylinders and pistons linked by separate circuits to a dual master piston. Each master piston also operates one of the rear brakes.
In an alternative arrangement, one master piston and circuit operates all four brakes and the other piston and circuit the front brakes only.
The two master cylinders for such systems are generally placed one behind the other (in tandem).
A dual system popular today has a front wheel and the rear wheel on the opposite side operating on one hydraulic system, and the other wheels on another independent system (below left).
The brake for Auto parking … and emergencies
The handbrake is normally used only after the driver has stopped the car by using the footbrake. Its other use is as an emergency brake, to stop the car if the footbrake system should fail.
The handbrake should be left on when the car is parked. Neglect of handbrake adjustment is dangerous, as the brake may fail to hold the car when it is parked on a hill.
Slowly releasing the handbrake and gently engaging the clutch while depressing the accelerator is an essential part of the technique of moving from rest up a hill.
On most cars, the handbrake operates only the rear brakes. It is a legal requirement that the hand control should operate the brakes mechanically (by rods or cables) if the pedal operates them hydraulically. When drum-type rear brakes are used, the same shoes can be used for both hand and foot control.
The handbrake lever may pull on a single cable, which is connected to a pivoted T-piece to transmit the pull equally to both rear brakes, or there may be two cables from the handbrake lever, one to each rear brake.
When disc brakes are used on the rear wheels, there are sometimes two pairs of pads straddling the disc, with one pair operated hydraulically from the pedal and the other pair cam-operated by the handbrake cables.
In yet another arrangement, small drum brakes, operated by the hand lever, are embodied in the rear disc brakes.
Auto Brake fluid and moisture
Brake fluid is hygroscopic and can absorb moisture from the atmosphere. Under heavy braking, water in the hydraulic fluid adjacent to the calipers or wheel cylinders boils when the operating temperature exceeds the boiling point of water under pressure. The water turns to steam which is compressible and may not permit the hydraulic system to retain pressure sufficient to operate the brakes effectively. This results in brake failure. Brake fluid takes up water in hot and humid conditions much faster than it does in a cool dry climate. Manufacturers recommend that brake fluid be drained and replaced at set intervals.
Disc brakes require the brake fluid to have a higher boiling point than the fluid for drum brakes. All brake fluid with the SABS mark has a boiling point suitable for disc brakes and can also be used for the drum brakes on older vehicles.
Engaging and releasing the Auto handbrake
The handbrake lever has a spring- loaded ratchet arrangement which allows the driver to set the brake with the required amount of tension between fully off and fully on. The lever is usually placed to the left of the driver, but it is sometimes on the right, between the seat and the door. An alternative is the ‘umbrella- handle’ control, placed beneath the instrument panel, with its release mechanism built into the handle grip.
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