Lotus Eleven/Elise/Exige. Manual - part 66

   Lotus Service Notes

                Section JJ

Page 17

Initial Movement

Initial movement of the brake pedal causes the input rod to move to the right, and allows the control valve

under the action of its spring, to close off the vacuum port in the driving piston.  The atmospheric port remains
closed.

      147b

Driving piston

Vacuum port
closed off

Brake pedal
input

Atomospheric
port closed

 

   Lotus Service Notes                                  Section JJ

Page 18

Pressure Balance

Further pressure on the brake pedal opens the atmospheric port and bleeds atmospheric pressure into

the left hand chamber.  This pressure imbalance causes the driving piston to move to the right, adding assistance
to the force applied directly to the output rod (and the master cylinder) via contact between the input and output
rods.  When the force produced by the pressure differential across the diaphragm balances the reaction force
of the main spring and master cylinder, movement of the driving piston ceases, with both control valve ports
closed, and the system once again in equilibrium.

From this position of equilibrium, further pressure on the pedal will tend to open the atmospheric port and

allow pressure in the left hand chamber to increase, and move the driving piston to the right before again
stabilising.  Any reduction in pedal pressure will tend to open the vacuum port and allow the higher pressure in
the left hand chamber to bleed off into the right hand chamber, whose depression is kept constant by its
connection with the intake plenum.  The pressure imbalance is reduced, and the driving piston will move to the
left under the action of the main spring, until equilibrium is again restored.

Higher pressure

Low pressure

balances manifold

from intake

pressure plus

manifold

spring

Vacuum port

Input from
brake pedal

Output to
master
cylinder

Atmospheric port

     j 47c

   Lotus Service Notes

                Section JJ

Page 19

Maximum Assistance

If the pedal is pressed hard, the atmospheric port will be held open, so that the left hand chamber will be

subject to full atmospheric pressure.  This is full servo assistance which will, with 0.8 bar of vacuum available
from the engine, increase the force applied to the master cylinder pushrod by approximately four times.

When pedal pressure is relaxed, the vacuum port in the driving piston will open and bleed the higher

pressure from the left side of the diaphragm into the right hand chamber and thence into the engine plenum,
collapsing the pressure differential across the diaphragm.  The driving piston is moved fully to the left by the
main spring.

The pliant reaction disc fitted between the input and output rods ensures a graduated application of servo

assistance and provides pedal feedback and 'feel' to the driver.

Atmospheric

Intake manifold

pressure

depression

Input from
brake pedal

Output to
master
cylinder

Contact between
input and output rods
via reaction disc

Atmospheric
port open

      j147d

 

   Lotus Service Notes                                  Section JJ

Page 20

Operational Check

As a quick check of servo operation proceed as follows:  With the engine stopped, press the brake pedal

several times to exhaust the servo unit of vacuum.  Keeping the pedal pressed (which should be 'hard' and
'high'), start the engine; The pedal should drop slightly as the servo vacuum builds up, and extra force is
produced.  If the pedal does not drop, it is most likely that there is a fault in the vacuum supply line.  Check the
vacuum hose, all connections and the non-return valve.  If the vacuum supply is not defective, the servo unit
should be replaced.
Setting brake pedal:  It is essential that the servo piston (and master cylinder piston) is allowed to return fully
when the brakes are released, and is not pre-loaded by mal-adjustment of the input pushrod.  See sub-section
JJ.11.
Stop light switch:  The stop switch is mounted in a right angle bracket fixed to the underside of the scuttle, and
abuts directly against the pedal.  The switch is retained in the bracket by a quarter turn mechanism.

To Replace Brake Servo Unit

1.

Remove the brake master cylinder (see sub-section JJ.9).

2.

From within the footwell, disconnect the servo pushrod from the brake pedal.

3.

Disconnect the brake servo vacuum hose, and release the four nuts securing the servo to the pedal box
extension plinth.  Withdraw the servo assembly.

4.

Replace the servo in reverse order to the above, tightening the servo mounting nuts to 25 Nm, and the
new master cylinder fixing nuts to 25 Nm.  Check pushrod adjustment (see sub-section JJ.11) and bleed
the hydraulic system.

Pedal box

Servo mounting bracket

Brake servo

to pedal box fixing

Servo to mounting
bracket fixing nut

         j191