Lotus 2004 Eleven Service Notes page 261

Lotus Service Notes
JJ.10 - VACUUM SERVO UNIT
The FTE brake vacuum servo is, with the exception of the air filter and non return valve, 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 vacuum non-return valve is incorporated into the vacuum hose
elbow connector in the front case of the servo unit, and a second in-line non-return valve is fitted into the supply
line close to the engine. 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 fitted between the brake pedal and master cylinder and is of the 'suspended in vacuum' type,
wherein a flexible diaphragm divides the space inside a steel shell into two chambers. The front chamber is
connected, via a non-return valve, to the vacuum produced in the engine's inlet plenum chamber. When the
brakes are 'off', this vacuum is also applied to the rear side of the diaphragm, which is held towards the rear of
the shell by a spring. The brake pedal is connected to the brake servo control rod, which operates a control
valve before pressing against the output rod via a pliant reaction disc.
Brakes Off
In the simplified schematic diagram, with the brakes off, the diaphragm and driving piston are pushed to
the left by the main spring, and the input rod is pushed to the left by its own spring, causing the control valve to
close off the atmospheric port. Engine vacuum admitted to the right hand side of the diaphragm is also
communicated to the left side of the diaphragm, and the unit is stable.
Brake pedal
connection
Air filter
Page 16
Rear chamber
Servo control rod
Diaphragm
Reaction disc
Section JJ
Diaphragm
Engine
vacuum
Front
chamber
Output
rod
Main
spring
j147a