Ford Explorer, Руководство по техническому обслуживанию и ремонту

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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 rear brakes. If the rear brakes 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 front brakes. In either case, the brake pedal moves farther when the brakes are applied,
and less braking power is available.
All dual circuit systems use a switch to warn the driver when only half of the brake system is operational. This
switch is usually located in a valve body which is mounted on the firewall or the frame below the master
cylinder. A hydraulic piston receives pressure from both circuits, each circuit's pressure being applied to one
end of the piston. When the pressures are in balance, the piston remains stationary. When one circuit has a
leak, however, the greater pressure in that circuit during application of the brakes will push the piston to one
side, closing the switch and activating the brake warning light.
In disc brake systems, this valve body also contains a metering valve and, in some cases, a proportioning
valve. The metering valve keeps pressure from traveling to the disc brakes on the front wheels until the brake
shoes on the rear wheels have contacted the drums, ensuring that the front brakes will never be used alone.
The proportioning valve controls the pressure to the rear brakes to lessen the chance of rear wheel lock−up
during very hard braking.
Warning lights may be tested by depressing the brake pedal and holding it while opening one of the wheel
cylinder bleeder screws. If this does not cause the light to go on, substitute a new lamp, make continuity
checks, and, finally, replace the switch as necessary.
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 apparent, the problem is inside
the master cylinder.
DISC BRAKES
Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems
utilize a disc (rotor) with brake pads positioned on either side of it. An easily−seen analogy is the hand brake
arrangement on a bicycle. The pads squeeze onto the rim of the bike wheel, slowing its motion. Automobile
disc brakes use the identical principle but apply the braking effort to a separate disc instead of the wheel.
The disc (rotor) is a casting, usually equipped with cooling fins between the two braking surfaces. This
enables air to circulate between the braking surfaces making them less sensitive to heat buildup and more
resistant to fade. Dirt and water do not drastically affect braking action since contaminants are thrown off by
the centrifugal action of the rotor or scraped off the by the pads. Also, the equal clamping action of the two
brake pads tends to ensure uniform, straight line stops. Disc brakes are inherently self−adjusting. There are
three general types of disc brake:
A fixed caliper. 1.
A floating caliper. 2.
A sliding caliper. 3.
The fixed caliper design uses two pistons mounted on either side of the rotor (in each side of the caliper). The
caliper is mounted rigidly and does not move.
HOW TO USE THIS BOOK
972 Basic Operating Principles