Exploded view of the brake light
switch
Hydraulic systems are used to actuate the brakes of all modern automobiles. The system transports the power required to force the 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 master cylinder consists of a fluid reservoir along with a double cylinder and piston assembly. Double type master cylinders are designed to separate the front and rear braking systems hydraulically in case of a leak. The master cylinder coverts mechanical motion from the pedal into hydraulic pressure within the lines. This pressure is translated back into mechanical motion at the wheels by either the wheel cylinder (drum brakes) or the caliper (disc brakes).
Steel lines carry the brake fluid to a point on the vehicle's frame near each of the vehicle's wheels. The fluid is then carried to the calipers and wheel cylinders by flexible tubes in order to allow for suspension and steering movements.
In drum brake systems, each wheel cylinder contains two pistons, one at either end, which push outward in opposite directions and force the brake shoe into contact with the drum.
In disc brake systems, the cylinders are part of the calipers. At least one cylinder in each caliper is used to force the brake pads against the disc.
All pistons employ some type of seal, usually made of rubber, to minimize 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.
The hydraulic system operates as follows: When at rest, the entire system, from the piston(s) in the master cylinder to those in the wheel cylinders or 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 to the wheel cylinders. Here, it forces the pistons outward, in the case of drum brakes, and inward toward the disc, in the case of disc brakes. The motion of the pistons is opposed by return springs mounted outside the cylinders in drum brakes, and by spring seals, in disc brakes.
Upon release of the brake pedal, a spring located inside the master cylinder immediately returns the master cylinder pistons to the normal position. The pistons contain check valves and the master cylinder has compensating ports drilled in it. These are uncovered as the pistons reach their normal position. The piston check valves allow fluid to flow toward the wheel cylinders or calipers as the pistons withdraw. Then, as the return springs force the brake pads or shoes into the released position, the excess fluid reservoir through the compensating ports. It is during the time the pedal is in the released position that any fluid that has leaked out of the system will be replaced through the compensating ports.
Dual circuit master cylinders 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 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.
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:
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.
The sliding and floating designs are quite similar. In fact, these two types are often lumped together. In both designs, the pad on the inside of the rotor is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly, bringing the outside pad into contact with the rotor. There are various methods of attaching floating calipers. Some pivot at the bottom or top, and some slide on mounting bolts. In any event, the end result is the same.
Drum brakes employ two brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular drum which rotates with the wheel assembly. The shoes are held in place by springs. This allows them to slide toward the drums (when they are applied) while keeping the linings and drums in alignment. The shoes are actuated by a wheel cylinder which is mounted at the top of the backing plate. When the brakes are applied, hydraulic pressure forces the wheel cylinder's actuating links outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are then forced against the inner side of the drum. This action forces the bottoms of the two shoes to contact the brake drum by rotating the entire assembly slightly (known as servo action). When pressure within the wheel cylinder is relaxed, return springs pull the shoes back away from the drum.
Most modern drum brakes are designed to self-adjust themselves during application when the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever, thereby causing rotation of the adjusting screw. Some drum brake systems are designed to self-adjust during application whenever the brakes are applied. This on-board adjustment system reduces the need for maintenance adjustments and keeps both the brake function and pedal feel satisfactory.
Virtually all modern vehicles use a vacuum assisted power brake system to multiply the braking force and reduce pedal effort. Since vacuum is always available when the engine is operating, the system is simple and efficient. A vacuum diaphragm is located on the front of the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is normally connected to the intake manifold by a vacuum hose. A check valve is placed at the point where the hose enters the diaphragm housing, so that during periods of low manifold vacuum brakes assist will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm and springs return the diaphragm and master cylinder pistons to the released position.
If the vacuum supply fails, the brake pedal rod will contact the end of the master cylinder actuator rod and the system will apply the brakes without any power assistance. The driver will notice that much higher pedal effort is needed to stop the car and that the pedal feels harder than usual.
Power brake systems may be tested for hydraulic leaks just as ordinary systems are tested.
| WARNING |
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Clean, high quality brake fluid is essential to the
safe and proper operation of the brake system. You should always buy the
highest quality brake fluid that is available. If the brake fluid becomes
contaminated, drain and flush the system, then refill the master cylinder
with new fluid. Never reuse any brake fluid. Any brake fluid that is
removed from the system should be discarded. |
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Exploded view of the brake light
switch |
On trucks equipped with speed control, the spacer washer is replaced by the dump valve adapter washer.
| CAUTION |
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Don't substitute any other type of retainer. Use only
the Ford specified hairpin retainer. |
| CAUTION |
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Make sure that the stop lamp switch wiring has
sufficient travel during a full pedal stroke! |
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Brake pedal position switch removal— F-150, F-250,
Expedition and Navigator; 1999-00 F-250SD, F-350, and F-Super Duty
Models |
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Master cylinder mounting—1997-98 F-250HD, F-350 and
F-SuperDuty |
To install:
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Detach the fluid level electrical harness
connector |
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If equipped with speed control, detach the brake
pressure switch electrical harness connector |
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Use a suitable size flare nut wrench to
... |
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... loosen the brake line fittings at the master
cylinder |
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Plug the brake lines and inlet ports of the master
cylinder |
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Remove the 2 retaining nuts and position the RABS valve
out of the way |
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Remove the 2 retaining nuts and ...
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... remove the master cylinder from the brake
booster |
To install:
To install:
| WARNING |
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Do not depress the brake pedal while the hydraulic
fluid supply line is disconnected. The inlet pressure port check ball may
be dislodged which will result in a non-functional backup system.
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To install:
| WARNING |
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All new master cylinders should be bench bled prior to
installation. Bleeding a new master cylinder on the vehicle is not a good
idea. With air trapped inside, the master cylinder piston may bottom in
the bore and possibly cause internal damage. |
Ensure the bleeding tubes are below the level of the brake fluid, otherwise air may get into the system making your bleeding efforts ineffective.
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Brake booster assembly— F-150, F-250, Expedition and
Navigator; 1999-00 F-Super Duty models with vacuum assisted booster
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To install:
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Hydro-Boost system on the F-Super Duty chassis
cab |
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Hydro-Boost system on the F-Super Duty commercial
stripped chassis |
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Hydro-Boost system on the F-Super Duty motor home
chassis |
| CAUTION |
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Do not depress the brake pedal with the master
cylinder removed! |
| CAUTION |
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The booster should never be carried by the
accumulator. The accumulator contains high pressure nitrogen and can be
dangerous if mishandled! If the accumulator is to be disposed of, do not
expose it to fire or other forms of incineration! Gas pressure can be
relieved by drilling a 1/16 in. (1.5mm) hole in the
end of the accumulator can. Always wear safety goggles during the
drilling! |
The system is, in effect, self-bleeding and normal vehicle operation will expel any further trapped
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Brake booster assembly—1997-98 F-250HD, F-350 and
F-Super Duty wit vacuum assist |
To install:
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Hydro-boost brake booster
assembly |
| WARNING |
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The power brake booster should not be carried by the
accumulator, nor should it ever be dropped on the accumulator. Check the
snap ring on the accumulator for proper seating before the power brake
booster is used. The accumulator contains high-pressure nitrogen gas and
can be dangerous if mishandled. |
To install:
| CAUTION |
|
If the accumulator is to be disposed of, it must not
be exposed to excessive heat. Before discarding the accumulator, drill a
1/16 inch (1.6-mm) diameter hole in the end of the
accumulator can to relieve the gas pressure. Always wear safety glasses
when performing this operation. |
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Brake booster assembly— F-150, F-250, Expedition and
Navigator; 1999-00 F-Super Duty models with vacuum assisted booster
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To install:
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Hydro-max brake booster
assembly |
Start the hydraulic fluid supply line before tightening the Hydro-Max nuts and bolts.
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Vacuum pump mounting—7.3L diesel
engine |
To install:
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Setting the correct indexing height for the height
sensing proportioning valve |
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A common height sensing proportioning valve—F-Super
Duty models |
If the linkage is disconnected from the valve, the proper setting of the valve will be lost and a new valve will have to be installed. The new valve will have the shaft preset and secured internally. If the shaft of the new valve turns freely, DO NOT USE IT!—The valve cannot be repaired or disassembled. It is to be replaced as a unit.—If the linkage is damaged or broken and requires replacement, a new sensing valve will also be required.
To install:
When servicing axle or suspension parts which would require disconnection of the valve, instead, remove the 2 nuts that attach the linkage arm to the axle cover plate. This will avoid disconnecting the valve and avoid having to replace the valve.
Metal lines and rubber brake hoses should be checked frequently for leaks and external damage. Metal lines are particularly prone to crushing and kinking under the vehicle. Any such deformation can restrict the proper flow of fluid and therefore impair braking at the wheels. Rubber hoses should be checked for cracking or scraping; such damage can create a weak spot in the hose and it could fail under pressure.
Any time the lines are removed or disconnected, extreme cleanliness must be observed. Clean all joints and connections before disassembly (use a stiff bristle brush and clean brake fluid); be sure to plug the lines and ports as soon as they are opened. New lines and hoses should be flushed clean with brake fluid before installation to remove any contamination.
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Use a brush to clean the fittings of any
debris |
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Use two wrenches to loosen the fitting. If available,
use flare nut type wrenches |
If the brake system is to remain open for more time than it takes to swap lines, tape or plug each remaining clip and port to keep contaminants out and fluid in.
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Any gaskets/crush washers should be replaced with new
ones during installation |
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Tape or plug the line to prevent
contamination |
To install:
| WARNING |
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Make sure the hose is NOT kinked or touching any part
of the frame or suspension after installation. These conditions may cause
the hose to fail prematurely. |
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Bleeder valve location—drum
brakes |
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Place a wrench onto the bleeder valve, connect a hose
and carefully open the bleeder valve to bleed the brakes—drum
brakes |
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Place a wrench onto the bleeder valve, connect a hose
and carefully open the bleeder valve to bleed the brakes—disc
brakes |
When any part of the hydraulic system has been disconnected for repair or replacement, air may get into the lines and cause spongy pedal action (because air can be compressed and brake fluid cannot). To correct this condition, it is necessary to bleed the hydraulic system so to be sure all air is purged.
When bleeding the brake system, bleed one brake cylinder at a time, beginning at the cylinder with the longest hydraulic line (farthest from the master cylinder) first. ALWAYS Keep the master cylinder reservoir filled with brake fluid during the bleeding operation. Never use brake fluid that has been drained from the hydraulic system, no matter how clean it is.
The primary and secondary hydraulic brake systems are separate and are bled independently. During the bleeding operation, do not allow the reservoir to run dry. Keep the master cylinder reservoir filled with brake fluid.