BRAKE SERVO : HAVE TO PUSH HARD THEN SUDDENLY WORKS

[alaric]

Hi.

It does sound like an air leak to me. I reason that when driving i.e. throttle open part way, there's not much vacuum in the manifold to pull a vacuum on the servo, so that if you have an air leak in the servo when you brake there's no or little servo action - in this situation the air leak rate is greater than the manifold vacuum pulling rate. But, while you are braking, there's a good vacuum in the manifold, so the manifold vacuum pulling rate is greater than the leak rate, i.e. the servo is being sucked on quite hard. It may take a little while for the servo to 'kick in', so to speak, while a vacuum is pulled on the servo diaphram. When you start braking there's extra air coming into the manifold from the servo diaphram, so you hear a slight change in not of the engine due to the additional air.

So I reckon it's an air leak, not the non return valve.

A test would be to try braking hard with your foot still on the accelerator - heel toe I think it's called as used for double de-clutch - try this carefully of course haha - you should find that the servo doesn't kick in as the vacuum in the manifold will still be weak.

I hope that helps.

Regards.

Sean.

[alan.barker]

Maybe a small hole in the Servo diaphragm. You can buy a Kit to overhaul a Servo.
Alan

[tonycharente]

Thank you Sean and Alan for your suggestions.
This afternoon I've been working on the Elan again. The other day I had unscrewed the airvalve cover to see if I could see anything obvious. This enabled me to fit jubilee clips onto the short rubber hose which links it to the servo. I had already also tighened the jubilee clip at the inlet manifold. Whilst removing the airvalve cover from this hose the diaphram fell off - which enabled me to see that there was some fairly thick grease on the "spike" of the diaphram support that goes down into the servo slave cylinder. It looked like the grease might be impeding the movement of the "spike", so I wiped most of it off. I then reassembled everything and took the Elan for another spin. Without being perfect it is very much improved. It's certainly no longer "all or nothing", even if not as smooth as a modern car. So I'm entirely happy with the result and will not be investigating any further.

The only thing is I don't know whether the improvement is down to removing air leaks or removing the thick grease, or both. I think it may be at least partially down to removing the links as my tick-over went up to nearly 1200 rpm and I had to adjust it back to where it was (1000 rpm).

Many thanks again,
Tony

[alan.barker]

Well done Tony,
Drive and enjoy
Alan

[pharriso]

Fantastic, I call that a result!
Pleased for you

[tonycharente]

FURTHER UPDATE
Having a 1957 Jaguar XK150 with a similar, (but bigger), Lockheed style servo and the same "all or nothing" servo action problem, I decided to tighten all of the numerous jubillee clips on all of its vacuum hoses BEFORE removing the airvalve cover. Same result as on the Elan - without being perfect it is very much improved. It's certainly no longer "all or nothing", even if not as smooth as a modern car.

As the servo on XK150's is even harder to get at than on an Elan, being mounted in a closed compartment way up behind one of the front wheels, I've not bothered attempting to remove the airvalve cover, being happy with what I have achieved. Therefore I don't know what impact, if any, removing the thick grease (see earlier post) achieves.
Thanks again to all,
Tony