Vacuum Servo Unit; Brakes Off; Initial Movement; Pressure Balance - Lotus Evora Service Notes

Lotus Service Notes
4. Release the two nuts securing the master cylinder to the brake servo and remove the cylinder.
5. Refit the master cylinder in reverse order to removal, using new locknuts and tightening to the following
torques:
- Master cylinder fixing nuts; 25 Nm
- Brake pipes to cylinder; 16 Nm
6. Fill the reservoir with DOT 4 non-mineral type brake fluid, and bleed the complete brake system of air
using the procedure in Sub-section JL.3.
JL.9 - VACUUM SERVO UNIT
The vacuum brake servo is secured to the front face of the pedal box via a machined alloy spacer, and is
operationally interposed between the brake pedal and master cylinder. Engine generated vacuum is used to
provide pneumatic assistance to the effort applied at the pedal. The unit is supplied by Continental Teves and
is a dual diaphragm unit combining a 178mm (7 in) and 203mm (8 in) diameter vacuum chamber into a single
compact unit to provide a 5:1 assistance ratio. With the exception of the air filter and vacuum elbow/non return
valve, the unit is a non-servicable sealed unit which if found to be faulty, must be replaced as an assembly.
The air filter (surrounds the input push rod) should be replaced whenever the brake system is overhauled, and
cleaned or replaced more frequently if the vehicle is operated in dusty conditions. A non-return valve is incor-
porated into the vacuum hose elbow connector in the front case of the servo unit. The elbow connector valve
is a push fit into a grommet in the servo shell, and is supplied complete with the grommet.
The servo is of the 'suspended in vacuum' type, wherein two flexible diaphragms divide each of two sec-
tions within the steel shell into two chambers. One pair of chambers, towards the front side of each diaphragm,
is connected via a non-return valve, to the vacuum produced in the engine's inlet plenum chamber. The second
pair of chambers, towards the rear side of each diaphragm, connect either to the front chambers, or to atmos-
phere, under the control of a double acting face valve.

Brakes Off

When the brake pedal is released, the diaphragms and driving piston are pushed fully rearwards by the
main spring, and the input rod (connected to the brake pedal) is also pushed rearwards by its own spring, caus-
ing the face valve to close the atmospheric connection. Engine vacuum admitted to front of the diaphragms is
also communicated to rear of the diaphragms via the face valve. The unit is in stable equilibrium.

Initial Movement

Initial movement of the brake pedal causes the input rod, input piston and face valve to move forwards,
closing off the front/rear communication.

Pressure Balance

Further pressure on the brake pedal pushes the input piston further forwards and opens the face valve to
bleed atmospheric pressure into the rear chambers. The resultant pressure imbalance across the diaphragms
acts to apply a force to the driving piston to assist pedal effort. When the combined input force from foot pressure
and servo pneaumatic pressure balances the reaction force of the main spring and master cylinder, movement
of the driving piston ceases, and both face valve ports close. The system is once again in equilibrium.
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Section JL