Page 591
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I
CONTENTS
500
GENERAL
516
BRAKE DISCS
517
BRAKE PADS
520
HYDRAULIC FOOTBRAKE SYSTEM
521
MASTER CYLINDER
522
BRAKE LINES
523
WHEEL CYLINDERS AND BRAKE HOUSINGS
524
BRAKE OPERATION
541
SERVO SYSTEM
551
HANDBRAKE SYSTEM
SAM!
Page 593
GENERAL
The car has two independent brake systems: the footbrake
or driving brake, which is hydraulic and acts on all four
wheels, and the handbrake or parking brake, which is mechanical and acts on the front wheels.
I
t
FOOTBRAKE OR DRIVING BRAK': SYSTEM
When the brake pedal ( 1) is depressed, it acts on two pistons in the master cylinder (3) via the servo unit (2); the
latter increases the pedal pressure and transmits the motion to the master cylinder which acts simultaneously
but independently on two hydraulic circuits to the front
(4) and rear (5) wheel brakes. Each circuit acts on a diagonally opposed pair of wheels, i.e. right front and left
rear, and left front and right rear.
The pressure is transmitted through the brake lines and
hoses to the wheel cylinders, where the pistons force the
brake pads against the discs (6). The braking effort is
greater on the front wheels, and this reduces the risk of
the rear wheels locking. If there is a leak in the system,
braking effort will be lost on one diagonal pair of wheels
only, the other pair being unaffected. The reduced pressure in the leaking circuit then acts on the brake warning
valve in the master cylinder and the brake warning light
on the instrument panel lights up.
As from model 1978, the master cylinder contains a float
which senses the level of the fluid in the container. In the
event of leakage, the float actuates a switch in the cover
of the brake fluid container and the brake warning light
will go on.
HANDBRAKE OR PARKING BRAKE SYSTEM
The action of the handbrake lever (7) is transmitted mechanically by a link mechanism and two wires to the
front wheel brakes (4) where a push rod acts mechanically on the pistons which force the brake pads against the
discs.
S4298
BRAKE SYSTEM
1. Brake pedal
2. Servo unit
3. Master cylinder
4. Brake housing, front wheel
5"4\11
.L
5. Brake housing, rear wheel
6. Brake discs
7. Handbrake lever
500-1
Page 594
MASTER CYLINDER
The master cylinder comprises a cylinder housing in which
two pistons, one for each brake circuit, transmit the force
from the push rod hydraulically. In model 1975-1977,
the leakage warning system consists of a piston in the
main cylinder which actuates an electric switch, in the
event of a pressure drop in one of the circuits, whereupon
the brake warning light will come on.
As from model 1978, a float mechanism in the cover of
the brake fluid container senses the level of the fluid in
the container. Should the level drop below a preset limit,
the movement of the float will actuate a switch in the
cover, whereupon the brake warning light will come on.
1
8
5
SECTION THROUGH MASTER CYLINDER,
1. Brake fluid reservoir
2. Primary piston
3. Secondary piston
4. Return spring, primary piston
5. Return spring, secondary piston
6. Brake warning piston
7. Brake warning switch
8. Stop pin
500-2
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MODEL 1975-1977
Page 595
1
SECTION THROUGH MASTER CYLINDER,
AS FROM MODEL 1978
1. Cylinder housing
2. Return spring, secondary piston
3. Secondary piston
4. Return spring, primary piston
5. Primary piston
6. Stop pin
Rest position
Braking
In the rest position, the. two pistons are held at the rear
end of travel by means of the return spring. The passages
between the brake fluid reservoirs and the two brake circuits are open.
When the brake pedal is depressed, the primary piston is
pushed forward by the push rod. This movement closes
the passage between the fluid reservoirs and the cylinder
and the fluid pressure in front of the piston rises. The increased pressure results in the secondary piston being
pushed forward. The same pressure acts on the front of
MASTER CYLINDER,
BRAKING,
S.AB
MODEL 1975-1977
500-3
Page 596
sure drop in the faulty circuit will give rise to a difference
in pressure which will cause the brake warning piston to
move towards the outlet on the primary side. The movement of the piston causes the pin from the brake warning
switch to be pressed into the switch, whereupon the electrical circuit to the brake warning light is closed.
both pistons and the pressure is transmitted to the brake.
In the event of a leakage occurring, for example, in the
primary circuit, the primary piston spring is compressed
until the piston acts on the secondary piston, and the
brakes in the secondary circuit function normally. In
master cylinders with a brake warning piston, the pres-
MASTER CYLINDER,
LEAKAGE
IN THE PRIMARY CIRCUIT, MODEL 1975-1977
The brake fluid reservoir comprises two mutually independent compartments. In the event of leakage in one
circuit, only one of the compartments will therefore be
emptied.
The fluid remaining in the undamaged circuit is sufficient
to drive the car safety to a workshop. As the two brake
circuits act diagonally, about half of the braking effect
always remains in the event of leakage in one .of the circuits. This also provides greater safety in steering the car
as one front and one rear wheel always run free and do
not lock.
In the event of leakage in the secondary circuit, the secondary piston is pushed forward, compressing the return
spring and coming into contact with the end of the cylinder. The reduced pressure in the secondary circuit
causes the brake warning piston to move towards thesecondary side, ana the brake warning switch lights up the
warning lamp. The primary circuit is unaffected.
In the event of a leakage occurring in a brake circuit in
cars with master cylinders containing a brake warning
float, the float will drop as the fluid level sinks, whereupon the switch in the cover will close the circuit to the
brake warning light.
MASTER CYLINDER,
500-4
LEAKAGE
IN THE SECONDARY CIRCUIT, MODEL 1975-1977
a.a.a
.J
Page 597
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WHEEL BRAKES
When the brake pedal is released, the piston seals in the
cylinder return the pistons to the rest position. Adjustment as a result of brake pad wear is thus automatic.
Each wheel brake assembly comprises a brake housing
with two pistons, a yoke and two brake pads. The front
brake housings contain the handbrake mechanism and
are securely mounted on the steering knuckle housings.
The rear brake housings are mounted to special mounting
points on the rear axle. The yokes slide in grooves in the
brake housings. When the brake pedal is depressed, the
fluid pressure is transmitted to both pistons in the cylinder. One of the pistons acts directly on one of the brake
pads while the other presses on the yoke which indirectly
transfers the pressure to the other brake pad. In this way,
the pressure will be the same on both sides of the disc.
Brake position
Rest position
6
PISTON SEALS, REST POSITION AND BRAKING
..,...
FRONT WHEEL BRAKE ASSEMBLY, LEFT
r
1. Yoke
2. Piston (indirect)
3. Piston (direct)
4. Handbrake lever
5. Brake pad (inner)
6. Brake pad (outer)
7. Cylinder housing
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i
SAAB
500-5
Page 598
Rearwheel brakeassembly, Girlingmake
Rearwheel brakeassembly, ATE make
Each wheel brake assembl_ycomprises a brake housing
with two pistons, a yoke and two pads. The yoke slides
in grooves incorporated in the body of the brake housing.
When the brake pedal is depressed, hydraulic pressure is
directed between the two pistons in the cylinder. One of
the pistons acts directly on one of the brake pads while
the other presses on the yoke which indirectly transfers
the pressure to the other brake pad. In this way, the pressure will be the same on both sides of the disc.
Each wheel brake assembly consists of a brake housing
comprising two parts which are bolted together. Each
half_is equipped with a brake cylinder. When the brake
pedal is depressed, both pistons act directly on the brake
pads, pushing th~m against the disc.
1
5
S4300
REAR WHEEL BRAKE ASSEMBLY, LEFT
1. Cylinder housing
2. Piston (indirect)
3. Piston (direct)
4. Yoke
5. Brake pad (outer)
· 6. Brake pad (inner)
500-6
REAR WHEEL BRAKE ASSEMBLY, LEFT
1. Cylinder housing
2. Piston
3. Brake pad
4. Retaining spring
SAAB
Page 599
HANDBRAKE MECHANISM
The handbrake mechanisms and their automatic adjustmentdevices
arecombined with two brake pistons. The
handbrake lever acts on a thrust plate which mechanically actuates the two pistons by means of a push rod. The
automatic adjustment device is built into the direct piston and consists of a sleeve with internal and external
threads. The push rod is screwed into the internal thread
and a drive ring fitted to a conical hole in the brake piston runs on the external thread which is specially de---signed with a coarse pitch.
When the brake is applied, the increase in hydraulic pressure between the pistons acts on the threaded end of the
push rod. As the other end of the push rod is not affected
by the pressure, the push rod and the direct piston will
be forced apart. The play between the push rod thread
3 12
and the internal thread of the sleeve even out and the
same applies to the external thread of the sleeve and the
drive ring. The pressure of the drive ring against the conical hole increases, and as a result of its special shape, the
sleeve turns in relation to the indirect piston and the push
rod. As the brake is released, the brake pistons are returned in the cylinder as a result of the seals. The sleeve
and push rod are returned by spring washers, the drive
ring makes contact with the other side of the thread on
the sleeve and the pressure against the conical seating is
reduced with the result that the drive ring revolves on
the external thread of the sleeve. When the pistons are
pressed further apart in the cylinder as a result of wear
on the brake pads, rotation of the sleeve in relation to
the push rod results in the latter being fed out, and this
ensures constant adjustment.
5
6
7
8
2
1
S 4307
11
ADJUSTMENT
4
10
DEVICE, HANDBRAKE
9
9
MECHANISM
1. Handbrake lever
2. Return spring
3. Brake piston (indirect)
4. Drive ring
5. Brake cylinder housing
6. Brake piston (direct)
7. Brake disc
8. Yoke
9. Brake pad
10. Sleeve
11. Push rod
12. Thrust plate
500-7
Page 600
SERVO UNIT
The servo unit is mechanioal.and'designed to increase the
pedal pressure when the1orake!lare:-aP,plied. The increase
in pressure from the servo unit is obtained from the vacuum in the inlet manifofdlof1tHe.'eng.ine. The inlet manifold is connected to the: servo unit by means of a hose.
The master cylinder is fitted between the brake pedal and
the brake master C}flinder and connected to these parts by
means of push rods. If a leak should occwr in the servo
unit, the two push rods then act as a simple push rod. The
brakes still work as usual, but considerably more pressure
on the brake pedal is required.
Braking
When the brake pedal is depres~ed, the push rod pushes
the valve piston and diaphragm forward, closing the bypass passage. The valve piston then opens a passage which
enables air at atmospheric pressure to flow through the
filter and to the rear of the diaphragm.
2
3
4
SERVO UNIT, BRAKING
The power boost is derived from the difference between
the vacuum in front of the diaphragm and the atmospheric pressure behind it. The maximum power boost is of
the order of 3:1.
SERVO UNIT, REST POSITION
1. Non-return valve
2. Push rod (master cylinder)
3. Seal
4. Return spring
5. Diaphragm
6. Valve piston
7. Filter
8. Dust cover
9. Push rod (brake pedal)
Rest position
In the rest position the diaphragm and valve piston are
held at the rear limit of travel by the return spring. The
same vacuum acts on both sides of the diaphragm since
the by-pass passage in the diaphragm is open.
500-8