Tandem Servo Section (Brakes Off); Maximum Assistance; Operational Check - Lotus Evora Service Notes

Lotus Service Notes

Tandem Servo Section (brakes off)

Main spring
Vacuum chambers
From this position of equilibrium, further pressure on the pedal will tend to open the atmospheric valve
face and allow pressure in the rearward chambers to increase, and move the driving piston forwards before
again stabilising. Any reduction in pedal pressure will tend to open the front/rear valve face and allow the higher
pressure in the rearward chambers to bleed off into the frontal chambers, whose depression is kept low by
connection to the intake plenum. The pressure imbalance allows the driving piston to move rearwards under
the action of the main spring and master cylinder, until equilibrium is regained.

Maximum Assistance

At around 900N of input rod pressure, the atmospheric port will be held open, so that the rearward cham-
bers will be subject to full atmospheric pressure. This constitutes maximum servo assistance which will, with
0.8 bar of vacuum available from the engine, increase the force applied to the master cylinder pushrod by a
factor of five.
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.

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 pro-
duced. 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.
Vacuum supply
Atmospheric
chambers
Section JL
Driving piston
Pliant reaction disc
Atmospheric Communication
Input piston
Vacuum communication
Atmospheric communication
Atmospheric
valve face
Input
rod
Air filter
Front/rear
communication
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