As of 2004 the Elise range has been using toyota engines instead of rover engines. At the same time brake assist was introduced by fitting a brake servo between the pedal push rod and the master brake cylinder. You can identify brake assisted models by the shape of the inspection panels below the windshield. With booster are higher and have a curved leading edge, without are flat, low and inclined.
Pressure difference between engine inlet manifold vacuum and ambient pressure is used to create an assisting force component boosting the brake input by the driver. The servo consists of a membrane (pressure difference x membrane surface = force) and some valves. With no brake input an internal valve is open and allows vacuum to enter both sides of the membrane, not creating a resulting force. With brake input, vacuum is closed for the input side of the membrane and ambient pressure is connected instead, while vacuum is maintained on the output side of the membrane. A set of springs close the input valve if brake pressure is released, and vacuum is re-installed on both sides of the membrane. The brake servo or brake booster is connected to the inlet manifold of the engine, using a vacuum line through the sill, provided with a check valve near the engine and another one at the connection to the servo itself.
Can this part fail occasionally? Apparently yes, but the brake pedal is always driving the master cylinder. It is just the power assist which may lack to some or full extent. I discovered that my booster was leaking internally because it was making noise (hissing and humming). A continuous flow of additional air into the vacuum line (red line in the graph) affected the idling of the engine as this air circumferes the Mass Air Flow sensor. (MAF)
How can you identify a failing booster without tell tale noises? Cut the engine while braking; if the pedal goes stiff immediately there is a leak ór both check valves have failed but that is nearly impossible. Normally it should take some pump brake cycles to level the vacuum in the booster until the pedal goes stiff.
How to replace the brake servo on the Lotus Elise or Exige?
– Have the aircon refrigerant be professionally evacuated and recycled.
– Remove the front wheelarch liners, door hinge covers and front clam.
– Lift the brake fluid reservoir from its support and put it aside (no need to disconnect or empty it).
– Remove the large hose between hvac box and air distribution box
– Remove the aircon pipe from condensor to dryer/receiver.
– Pull the check valve connector from the rubber grommet.
– In the footwell, remove the split pin from the pedal push rod and release the pedal
– Remove the forked connector from the push rod.
– With a shortened spanner, remove four nuts at the back of the booster, leave the alu plinth in place. Don’t drop anything 😉
– Remove the nuts connecting the master cylinder and move the master cylinder away from the booster (there is limited room without the need to open the brake circuit).
– Subtract the booster.
Fitment of the replacement booster in reverse order. Pay attention to the correct pedal height and brake light operation. Do not forget to secure the pedal with a fresh split pin and lock the nut to the forked connector.
I used the opportunity to install a properly painted second hand booster as they corrode in no time.