Brake System
The vehicle is fitted with four braking systems:
The braking system is a hydraulically operated system with ventilated discs all round. Twin piston aluminum alloy calipers act on the front discs and single piston aluminum alloy calipers on the rear. A mechanical parking brake function is operated by a parking lever mounted on the passenger seat frame. An electrically powered pump provides vacuum to a servo which assists the operation of a tandem master cylinder. Fluid from the master cylinder flows through front/rear split hydraulic circuits to the calipers via the ABS modulator and a combination of solid and flexible pipes.
The regenerative braking available can produce a similar braking effect to a gasoline engined cars with manual transmission. Sensible driving, which predicts braking requirements and avoids the need for “panic braking” using the friction brakes, will significantly increase the driving range of the vehicle. For further information, refer to this information.
The vehicle is fitted with twin hydraulic circuits. The lower portion of the fluid reservoir has a separator which divides the fluid into two chambers. Each chamber feeds fluid to one circuit so that, in the event of fluid loss from one circuit, the other circuit would continue to operate normally. This will provide some braking effect although it will be less effective than with both circuits operational. Symptoms of single circuit failure include increased stopping distances and pedal travel when the brakes are operated.
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WARNING: DO NOT continue to drive a vehicle with one failed hydraulic circuit: braking will be severely reduced. The twin circuit design has been devised to provide an emergency backup only. In the event of circuit failure, stop the vehicle as safely as possible and repair the problem before continuing to drive. |
The pedal box is fitted under the dash board and is secured to the body tub. An offset crank arm on the brake pedal pivot connects the pedal to the servo input pushrod. The two components are linked by a clevis pin which is held in place by a split pin.
The brake light switch bracket is riveted to the pedal box and provides a bayonet socket into which the switch can be inserted. Rotating the switch 90° clockwise locks the switch in position. A four-pin harness connector provides a 12V feed and earth path (two of the pins are redundant).
The switch consists of a casing, a spring loaded plunger and two contacts. The end of the plunger rests against the back of the brake pedal which holds the plunger in against the spring pressure. As the pedal is depressed, the plunger is forced out. If pedal movement is sufficient, the plunger will close the switch contacts, completing the circuit and illuminating the brake lights. The same circuit is monitored by the ABS and cruise control systems.
Brake Fluid Reservoir and Low Level Fluid Sensor
The brake fluid reservoir is mounted remotely from the master cylinder to allow clearance for the hood. Two rubber hoses connect the reservoir to each side of the master cylinder using push fit connections and spring clips.
The translucent plastic brake fluid reservoir allows the brake fluid level to be checked using the level markings on the outer surface. A fluid level switch is incorporated into the reservoir cap which is screwed onto the top of the brake fluid reservoir. The assembly comprises of electrical contacts, a plunger, a float and a flexible grommet.
The brake master cylinder is a non-serviceable conventional tandem type unit mounted onto the front face of the servo on two studs and secured with nuts. Two internal pistons separate the independent front and rear brake circuits. Two larger diameter pipes transfer brake fluid from the master cylinder to the ABS modulator.
The ABS modulator is a non-serviceable unit consisting of an integrated hydraulic modulator and ABS ECU. The modulator, fitted on rubber mounts on the front LH side under the hood, is fed via two brake pipes, one from either side of the split master cylinder. Four brake pipes exit the valve block and feed through to each brake caliper.
The brake servo assembly is of a standard, suspended vacuum, single diaphragm type fitted to the body secured with three nuts. The circular housing contains diaphragms, a driving piston, a control valve assembly, input and output push rods and a non-serviceable filter. The input push rod is connected to the brake pedal and passes through the driving piston to the control valve assembly. The output push rod locates in the primary piston of the master cylinder. A port in the front face of the servo housing is connected to a vacuum line from the vacuum pump via an elbow joint. A non-return valve is incorporated into the elbow joint.
Throughout the vehicle, solid 0.186 in (4.76 mm) brake pipe is used, secured to the body using clips. The flared pipe ends are fitted with metric unions which mate with conical seats machined into components. Brake pipes run along the LH and RH bodysides to each rear brake.
Flexible rubber hoses connect the solid brake pipes to the brake calipers. Steel spiral wrap is fitted to the brake hoses to prevent chafing against suspension components. The brake hoses are secured to the upper wishbones at the front using a single threaded stub and the lower wishbones using a single threaded stub at the rear.
Description | Specification |
Brake fluid | Any brand of DOT 4 brake or brake/clutch fluid |
Capacity (fill from dry) | 1.6 quart (US)/1.5 l |
DOT 4 brake fluid is specified for use in the hydraulic braking circuit. DOT 4 brake fluid is an incompressible glycol based fluid and is used to transfer brake pressure from the master cylinder through the brake pipes to the brake calipers. Brake applications generate heat energy which is transferred into the braking components. Therefore brake fluid must have a high boiling point to remain efficient. Brake fluid is hygroscopic, meaning that it absorbs moisture from the atmosphere. The dry boiling point of brake fluid, when new, is 446°F (230°C); the wet boiling point of brake fluid, that is when it is in use and has absorbed moisture from the atmosphere, is 311°F (155°C). If the brake fluid boils, compressible gas bubbles form resulting in a longer pedal travel, potentially reducing braking effect. This is one of the causes of brake pedal fade.
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CAUTION: DO NOT use fluid from unsealed containers as it may have absorbed moisture from the atmosphere. |
It is essential that the correct specification of brake fluid is used. Incorrect specification brake fluid can cause damage to the seals within the hydraulic system and lead to brake failure.
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CAUTION: DO NOT use synthetic brake fluid. |
The front calipers are of fixed design and are constructed from two steel bodies secured together using four bolts. They are radial mount calipers secured to the rear of the forged suspension upright using two bolts. Each caliper incorporates two pistons hydraulically connected using a link pipe. A seal at the outer end of each piston prevents the piston, seal and bore becoming contaminated, damaged or corroded by road dirt. A single bleed nipple and dust cap is fitted on the outside face of the caliper.
Within each piston bore, a specially shaped groove is machined to provide a recess for the hydraulic seals. These have several functions:
The rear calipers are of floating type and are constructed from steel mounted on the rear upright using two bolts. Each caliper incorporates a single piston. A seal at the outer end of the piston prevents the piston, seal and bore becoming contaminated, damaged or corroded by road dirt. A single bleed nipple is fitted on the outside face of the caliper.
The calipers feature an integral parking brake mechanism connected to the parking brake cables via steel eyelets. The calipers feature a pair of steel discs featuring machined ramps. Ball bearings locate in the ramps.
Description | Specification (Front) | Specification (Rear) |
Disc diameter | 11.81 in (300 mm) | 12.20 in (310 mm) |
Disc thickness – new | 1.02 in (26 mm) | 1.02 in (26 mm) |
Disc thickness – service limit | 0.98 in (24.9 mm) | 0.98 in (24.9 mm) |
Both front and rear brake discs are handed rotors of cast iron construction with a ventilated design and are fitted to the hub bearing drive flange secured by a single countersunk screw.
The ventilation openings all face forwards to improve the internal flow of air through the curved cooling vanes. Cross drillings in the discs reduce the amount the pad faces harden or 'glaze' in use. Over the life of the pads this gives a more consistent braking effect as the friction surfaces wear since glazed pads have a reduced coefficient of friction. The drillings also prevent the build up of gas between the brake pads and discs, which can contribute to brake fade during prolonged or heavy braking.
Re-surfacing of the brakes discs is not permitted and discs must be renewed in axle pairs in the event of excessive runout, scoring, ridges in the braking surface, cracking or corrosion.
Description | Specification (Front) | Specification (Rear) |
Brake pad thickness, excluding back plate – new | 0.354 in (9 mm) | 0.256 in (6.5 mm) |
Brake pad thickness, excluding back plate – service limit | 0.098 in (2.5 mm) | 0.098 in (2.5 mm) |
The front and rear brake pads are manufactured from both lead and asbestos free materials.
The front pads are retained in the caliper body using two retaining pins and a directional anti-rattle plate to reduce brake pad squeal. Each half of the caliper body has recesses on the inner faces to locate the brake pads.
The rear brake pads are retained by a single retaining pin in the caliper body. An anti-rattle spring is also fitted to the rear caliper.
NOTE: Apply a light coating of any proprietary brand of high temperature copper anti-seize paste to the contact area between the pad and caliper piston.
With gasoline internal combustion engines, a vacuum forms in the inlet manifold which can be utilised by the servo to provide brake pedal assistance. On electrically powered vehicles no inlet vacuum is produced therefore an electrically powered vacuum pump is used.
The pump assembly non-serviceable unit encased in a hard plastic outer casing which is totally enclosed in a rigid NVH (Noise, Vibration and Harshness) package and mounted to a bracket via three rubber isolators. The complete assembly and bracket is then bolted to the crash structure under the LH inner wing liner. The pump consists of an electric motor which drives two vertically opposed pistons and diaphragms to provide improved balance and reduced vibration. Both pumping chambers are linked by a single tube and air is drawn from the servo through a fibre reinforced rubber hose which prevents collapse due to atmospheric pressure. Non-return valves are fitted to each chamber to prevent leakage. The pumped air is exhausted through a machined exit port via a non-serviceable gauze filter.
The motor is continuously powered by a 12V supply from the switchpack with the key in the ON position and is controlled by dedicated sensors independent of the vehicle's ECUs. The two sensors are mounted onto a Printed Circuit Board (PCB) which is mounted onto the outside of the pump's lower vacuum chamber. The pressure sensing bulbs extend through machined holes into the lower pressure chamber to measure vacuum directly. They are sealed in place with an O-ring to prevent leakage. The maximum vacuum sensor switches power off if a pre-determined level of vacuum is exceeded; the pressure switch starts the pump if vacuum is insufficient.
The wheel speed sensors are active sensors (i.e. they receive a power feed from the ABS ECU) that supply the ABS modulator with wheel speed signals. A wheel speed sensor is installed in each wheel bearing unit in close proximity to the inboard seal. The seals, which rotate with the wheels, each contain magnetic elements arranged in pole pairs. When the wheels rotate, the pole pairs in the seals induce sinusoidal voltage fluctuations in the wheel speed sensors that are converted into square wave signals and output to the ABS modulator. The frequency of the signal is proportional to wheel speed. Each wheel speed sensor has a fly lead connecting it to the wiring harnesses.
The parking brake is bolted to the passenger seat frame using three ‘patch lock’ type bolts. A conventional thumb operated pawl and ratchet release button is incorporated for locking and unlocking the lever. A parking brake switch is fitted to the lever, secured with two screws.
A single cable is attached to the lever and extends from the rear of the lever under the vehicle to a compensator. The cable is attached at either end using a clevis pin and an R-clip. A second, longer cable is attached to the parking brake mechanism on each rear caliper. The cable passes through an outer cable section between the caliper and brackets on the rear crossmember and also through the compensator. The outer cable section on each side is secured to the front of the lower wishbone using two lugs. Cable adjustments can be made at the threaded section of the outer cable where it passes through the bracket on the LH side of the rear crossmember.
Each rear caliper contains a self-adjusting mechanical link to the rear of the caliper pistons which are pushed out to apply the brake pads to the rear discs when the parking brake is operated.
When the key is turned ON, the Anti-lock Braking System (ABS) ECU performs a check of the brake related warning lamps on the instrument pack as part of the power up procedure. The warning lamps are illuminated for approximately five seconds and then extinguished. If a fault warning lamp remains illuminated after the lamp check, a fault has been detected and repair action is required.
NOTE: The ABS warning lamp is also used to indicate “Parking brake on” and “Low brake fluid”. To be able to see the warning lamp carry out the five second ABS self check, the parking brake should be off. Ensure the vehicle cannot roll during such a test.
When the brake pedal is pressed, the movement of the master cylinder piston produces hydraulic pressure which passes through the brake pipes and hoses to the calipers. The front/rear split braking circuit ensures that braking is still available to one circuit albeit at a reduced level with increased foot pedal travel if one circuit of the system suffers hydraulic failure.
NOTE: Although this component is generally referred to as a vacuum servo, only a partial vacuum is formed in either the pump or the servo 14.5 psi (1 bar) is atmospheric pressure, 0 psi (0 bar) is full vacuum.
The brake servo assembly provides power assistance to reduce the pedal effort when braking. If the brake servo assembly fails, the hydraulic system still functions but will require greater brake pedal effort due to the lack of vacuum assistance.
The servo only operates when a vacuum is available. This generally means when the key is in the ON position and the vacuum pump is operational. However, due to a Non-Return Valve (NRV) in the servo vacuum hose, a minimum reserve of vacuum will temporarily be available with key in the OFF position or if the vacuum hose or pump fails.
The brake servo pump produces a servo pressure approximately 10 psi (0.7 bar) which produces approximately four times the force at the output push rod than was exerted on the input push rod by the brake pedal.
In normal driving the servo pump provides vacuum to both sides of the main diaphragm. However, when the brake pedal is depressed, the input push rod moves forward causing the vacuum port, connecting the two sides of the diaphragm, to close. Continued movement opens an atmospheric port allowing air to enter the rear chamber through the air filter. The difference in air pressure between the front and rear sides of the diaphragm causes the diaphragm and output push rod to move forwards acting on the master cylinder with increased force, thus assisting the driver.
When the brake pedal is released, springs within the servo return the push rods and diaphragm to their original position. As this happens the atmospheric port is closed and the vacuum port opened. Air is then drawn through the vacuum pipe by the pump and vacuum is returned to both sides of the diaphragm.
In the event of servo failure, the input push rod can be pushed forward into contact with the output push rod. This provides a direct link between the brake pedal and the master cylinder.
NOTE: Greater effort (approximately four times as much) will need to be applied to the brake pedal to achieve the same braking effect if the servo is not operating correctly.
Low Level Fluid Sensor - Test Button
If the brake fluid level in the master cylinder is within its minimum and maximum levels, then the float should be immersed in the brake fluid which will force the plunger upwards, breaking electrical contact between the two terminals of the switch. A flexible grommet acts as a sealing cap at the top of the switch and also offers resistance to limit the upward movement of the rod/plunger. If the level of brake fluid falls below a pre-determined level in the reservoir (due to fluid loss or excessive fluid displacement caused by brake pad wear) then the float will no longer be supported by the fluid and the plunger will fall and close the switch contacts. Closing the contacts will complete the earth circuit from the brake warning lamp in the instrument pack to earth and illuminate the warning light.
This circuit can also be manually checked without disturbing the reservoir cap by pushing down gently on the grommet in center of the cap switch which will close the contacts and illuminate the brake warning lamp (the key must be in the ON position for this test).
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CAUTION: The brake pads are the only serviceable items on the brake calipers. DO NOT loosen or remove the fixing bolts, link pipe, seals or pistons. |
The hydraulic pressure generated by the master cylinder is transferred along the brake pipes and hoses to the calipers. The hydraulic pressure is directed to the rear of the caliper pistons. The piston is pushed forwards and the brake pad applies pressure to the brake disc.
On the rear brake calipers, the hydraulic pressure is applied to the single inner piston and brake pad. The reaction force slides the calipers on the guide pins against the direction of the piston movement applying pressure to the brake disc from the outer brake pad.
The friction between the brake pad and discs slows the vehicle converting the kinetic energy to heat energy.
Anti-lock Braking System (ABS)
The ABS function prevents the road wheels locking during heavy brake applications, thus maintaining vehicle stability and enabling the driver to steer the vehicle even under emergency conditions. During braking an increasingly larger proportion of the vehicle's weight is transferred from the rear wheels to the front wheels. The ABS modulator lowers the braking pressure to the rear wheels preventing over braking and a possible skid situation.
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WARNING: ABS is an aid to retaining steering control and stability
while braking:
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NOTE: During normal braking the feel of the brake pedal on vehicles equipped with ABS will be the same as that on non ABS vehicles. During anti-lock braking operation the driver will experience feedback in the form of a pulsating brake pedal and solenoid/pump motor noise from the ABS modulator.
When the brake pedal is pressed, pressurised fluid from the master cylinder assembly flows through the normally open isolation valves to operate the brakes. When the brake pedal is released, springs in the master cylinder return the pistons to the rest position and the hydraulic seals in the calipers draw in the caliper pistons. Brake fluid is forced back along the brake pipes to the master cylinder and the pads no longer apply braking force to the discs.
The ABS function is automatically enabled whenever the key is in the ON position. Self monitoring for faults is continuous and if the ABS ECU detects a fault it illuminates the instrument pack warning lamp and stores a Diagnostic Trouble Code (DTC).
Under normal braking, the ABS ECU constantly checks the relative speeds of the four wheel sensors. If the ECU determines that one wheel is decelerating more quickly than the other wheels it will interpret this as slippage. If slippage passes a pre-determined level the ABS ECU will determine that ABS braking is necessary.
To activate ABS braking the ECU energises the isolation and dump valves of the related brake and starts the return pump. The isolation valve closes to isolate the brake from pressurised fluid; the dump valve opens to release pressure from the brake into the accumulator and the inlet side of the return pump. The brake releases and the wheel begins to accelerate. The ECU then operates the isolation and dump valves to control the supply of hydraulic pressure to the brake and apply the maximum braking effort, for the available traction, without locking the wheel.
The parking brake operates on the rear wheels only. When the parking brake lever is raised, the front cable is pulled forwards. This, in turn, pulls the compensator connected to the center of the rear cable. The rear cable connects to levers on each caliper which operate the pistons using mechanical force. Pulling the parking brake lever rotates each lever causing rotation of one of the pair of steel discs within the caliper. Rotation of the second disc is prevented by a stop bolt in the cylinder. Ball bearings located in ramps machined in the discs, force the discs apart applying force to the piston via a screw and nut with steeply pitched threads.
The nut is held in the piston by a one way clutch, which is locked when the parking brake is applied. The nut is allowed to turn when the mechanism is released, or when the piston is operated hydraulically by the footbrake. This allows automatic adjustment of brake pad wear as the nut in the piston screws further along the thread.
While the parking brake is applied, the parking brake pawl operates the switch connecting an earth to the instrument pack. If the key is in the ON position, the brake warning lamp illuminates.
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CAUTION: The automatic adjustment functionality of the rear caliper relies on the caliper lever being able to return to its fully released position when the parking brake is off. Take care to follow the correct procedure in the Service Manual when adjusting the parking brake cable. |
NOTE: When fitting new rear brake pads it is necessary to rotate the pistons to screw them back down the thread. Without this rotation the pistons will not retract sufficiently to allow the new pads to be fitted. Always refer to the correct procedure in the Service Manual.