Lotus Elan

Faulty +2 servo?

PostPost by: frogeyesimon » Mon Jan 19, 2015 2:07 pm

My +2 has a Lockheed servo. I have rebuilt the master cylinder, bled the system, have no leaks, good discs & pads and a firm pedal.

When I gently apply the brakes at low speed, very little happens, then as I increase the pressure on the pedal, the brakes grab and the car stops abruptly. This isn't noticeable in the same way at higher road speeds as the tendency is to press the pedal harder anyway.

When braking from a higher speed and gently easing off the pedal pressure as the car comes to a halt, the brakes seem to behave correctly.

The standard servo test of switching the engine off with the brake pedal depressed results in the brake pedal sinking slightly - I believe this to be the correct sort of behaviour.

Could my symptoms be indicative of a faulty servo or is it more likely to be something else like a sticking caliper? - Any suggestions would be welcome.

Simon
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1968 +2S 130 50/1189
1959 Frogeye Sprite
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PostPost by: worzel » Wed Jan 21, 2015 9:42 am

Hi Simon

Might be worth looking at the air valve in the top of the servo. It's a small diameter piston, in the hydraulic circuit and is located under the air filter assembly. To access, remove the snap on cover that holds the filter, and (I'm going from memory now as I removed the servo on my car years ago), undo the six phillip headed screws and after removing the plastic filter base you should see a metal plate that is held to the metal body of the servo by 4 Phillips headed setscrews. In the very centre of this plate is a small diameter hole and under that hole is small piston. When you press the pedal hydraulic pressure forces this piston up slightly which in turn allows vacuum to move from one side of the servo chamber to the other.

These pistons often stick- usually resulting in the brakes staying on for a second or two after releasing the pedal. I'd check if the piston is free to move by pushing it down a bit (leave the covering metal plate in place otherwise the piston will shoot out) then get a helper to press the pedal and see if the piston moves up against the underside of the retaining plate. If it's stuck/sticking you might try removing the plate and getting a helper to press the pedal and eject the piston (keep the pedal pressed down to avoid air ingress), clean the piston and the piston bore in the servo and then carefully replace the piston whilst your helper very gently presses down on the pedal. This way you might avoid having to bleed the whole system.

Regards

John
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