leaky servo - Drew20
another question about my golf...
I've just retro fitted a rev counter and one thing has become apparent.
When my foot is on the brake pedal the engine idles about 50rpm slower than when my foot is off the brake pedal. It does this all the time and not just when the vacuum is being restored in the servo.
Does this mean I have a leaky servo?
if so is it leaking when brake pedal is up or down?

leaky servo - Andrew Moorey (Tune-Up)
Pressing the brake pedal allows air into the manifold as it recharges the vacuum in the servo so it sounds like your servo is ok.Prove this by pumping the pedal and onthe golf you will temporarily lose #4 cylinder until the servo vacuum is replete.

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OK, do servos work like this...? - Drew20
shouldn't the revs come back to normal once the vacuum is restored? or have I missed the point?
brake pedal pressed, vacuum behind the servo diaphragm is vented but vacuum in front of diaphragm is sealed so no air is let into manifold.
foot lifted off pedal and chamber behind servo is sealed to atmosphere again and seal between the two chambers is broken, vacuum is restored behind the servo once again allowing a small amount of air into manifold temporarily.

when pressing the pedal the revs are constantly lower and do not recover until the pedal is released. I thought no air could enter the manifold system until the pedal is released and then only very temporarily

thanks Andrew
OK, do servos work like this...? - james_60

the engine is creating vaccuum for the servo thus lowering the engine speed if you are on the go as in the car is moving you wont notice the lowering of engine revs but it is normal if the car is at a stand still as the car is idling

J Stephenson

p.s if you think your car has a problem then take it to a professional but as others have suggested there isnt a problem then dont worry
OK, do servos work like this...? - Drew20
thanks guys, I have no problem with the engine revs varying slightly. I was concerned that the symptom I described might indicate a problem with the brakes system, obviously not as everyone says this is normal.

I just can't see why pressing the brakes should create a continuous leak in the manifold vacuum. When stood at lights you press the pedal; the revs drop imediately and stay down until the pedal is released (whether 2 secs or 10mins). Seems odd to me but if that's what its supposed to do then my question is answered, thanks
OK, do servos work like this...? - Plantman
Well out of interest my \'94 RS2000\'s revs actually increase when i pump the brake pedal.
they go up from 800 to just over 1000.

if you just hold the brake on nothing happens. why is this?

i can only assume that the revs increase to help the braking effort and abs?

OK, do servos work like this...? - Drew20
ah OK, to step back a bit.
on my banger golf I was assuming that the revs changed because when the brake was pressed and/or released I was introducing a small but continous leak into the manifold which affected the fuel mixture slightly (but continuously).
This theory came from my limited knowledge of how brake servos work, but apparently my cars symptoms are not abnormal.

I would expect a small temporary leak as each time you release the pedal the vacuum in one chamber of the servo has to be restored.
my original question relates to the fact that my revs alter indefinitely depending on whether I am pressing the brake or not.

you say that holding your brake on does not affect revs, this is what I would expect (the small 'leak' as the servo replenishes is very short lived and not noticeable).
However, pumping the brakes means that the servo is having to replenish its vacuum almost continuously, ie you are introducing a 'continuous' leak into the manifold vacuum which is affecting fueling and therefore revs
OK, do servos work like this...? - none
Drew20, Agree with you, sounds like a leak. Should be easy enough to disconnect and blank off the servo pipe (thumb ?) See what happens to the revs then. It's a VW though and might have some odd connection between the brake light switch and the fuel system.
OK, do servos work like this...? - Aprilia
Pressing the brake pedal would produce a momentary change in idle speed as the vacuum is replenished - it should not produce a continuous change.
I think you do have a leak somewhere, probably within the servo unit itself. However, initial things to check would be one-way valve in the servo pipe and rubber seal at the 'elbow' where the pipe goes into the servo.

Test the servo seals by running the engine for a short while (without touching the brakes). Turn engine off and them press on the brakes several times. You should feel the servo assistance intially, but wearing off (pedal becoming harder) after a couple of presses as you exhaust the vacuum.


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