Brake Pedal Push Back

[mark030358]

Had a look at the archives and this subject has been discussed, but never answered apart from "swap the servo".

So one of the downsides today rre my run to the tuners was that my brake pedal started to "Push Back" up and go really hard, but the condition almost immediatley rectified itself. The condition came and went on my hours drive home but the brakes appeared to work just fine. If you have experienced the brake pedal pushing back it really is a weird feeling.

So my thoughts are that for the pedal to rise, fluid must be being pushed back from the servo to the master cylinder. From recollection it happens if I jut touch the brake, so as I release or gently maintain the brake pressure (I "think" maintain, need to try again) up comes the pedal. So the fluid lifting the pedal must be being moved by the servo and I am wondering if the control piston is sticking with the vacuum port on the "T" valve still open as per the "Brake Released" image attached, combined perhaps with a passing seal on the control piston. I cant see it being a passing seal on the output piston as not sure how this would move fluid back to the reservoir.

Anyway, one thing for sure the servo is coming off and I might just do all the caliper seals while I'm at it.

cheers
Mark

[Simmo78]

Hi Mark, I’ll be following your exploits closely.
As like you over the past year I’ve had the same experience, although the brake pedal on my S4 would come back with such force it was impossible to then push it back down and unfortunately the brakes would be locked on and the car immobile. The only way to move the car / brake pedal was to crack off a bleed nipple.
I’ve spoken to many lotus people and brake caliper / servo re-manufacturers and nobody can give a definitive explanation.
My problems existed even after having had the servo refurbished by a reputable company.
I have recently rebuilt all the calipers with new seals and pistons, renewed the flexible hoses and have only just replaced the master cylinder. However one thing I did note was that whilst the master cylinder was recently overhauled by myself the small rubber seal at the end of the piston was a really tight fit / swollen and would often stick inside the white plastic cup resulting in the brake fluid not being able to return to the reservoir - hence replacing it with a new item.

[Hawksfield]

Hi its the control piston primary seal causing the lock up in girling servo. Renew seals
John

[nmauduit]

I find this article from MG enthusiasts quite clear on servo operation, which in turn may help diagnosing the source of a malfunction

http://mgaguru.com/mgtech/brakes/bt201k.htm

[Phil.C60]

It is, as John says, almost certainly a problem with the control piston getting stuck. The last issue I had like this was with a Rover P6 V8. Slightly different servo arrangement but the principle is the same: The piston bore and piston were rusty/corroded, probably due to water in the brake fluid making the piston a very tight fit in the bore. The brake fluid pressure from the pedal was sufficient to move it and operate the servo, but it wouldn't return on it's own so held the air admittance valve open causing the brakes to drag and occasionally lock on at traffic lights. Customer complaint was "occasional lack of power pulling away"! The servo had already been supposedly "Reconditioned" but there was no real access to the control piston bore so despite the rest of it looking lovely, it just didn't work. Replaced with NOS unit and the car was fine.

[mark030358]

Thanks to all posters. So we agree the control piston is the culprit….

As I said I the servo will be coming off and going for an overhaul. Preferably I’d like to sleeve both the bores, but not sure if the control piston bores can be done. Anyway I’ll be giving “PastParts” and the guys who advertise in Club Lotus a call on Tuesday to see who does what.

Cheers
Mark

[alan.barker]

Mark,
Is there enough freeplay at the Master Cylinder and not the wrong length Pushrod.
Alan

[mark030358]

Mark,
Is there enough freeplay at the Master Cylinder and not the wrong length Pushrod.
Alan

Hi,
Yes MC all OK and the brakes have been fine for +15 years, definatley the servo on this occasion. Given the service interval is 3 years, guess I have done quite well.

thanks
Mark

[richardcox_lotus]

Mark, in 29 years ownership I’ve experienced 3 servo failures, all showing the same symptoms as you describe.

It’s not been even length of time for them though. Lack of use makes the situation worse.

Regards
Richard