Servo fault

Hi all

I noticed the other day when i held my foot on the brake pedal before starting the engine the pedal didnt sink. At it alarmed me that its a servo fault.

Today ive done the same again this time to actually test it. And the pedal doesnt sink at all when the engine is started untill the pedal is pumped.

Ive pulled the vacuum hose off the servo and there appears to be decent suction. And theres no obvious cracks in the servo.

The reason im posting this is because it seems odd that i get servo assistance once the pedal is pumped or moved after the engine has been started.

How can i determine if its servo or vaccum pump? Thanks

Start the engine and pull the vacuum line out of the servo, should be a good bit of suction.
I just had the same thing, my vacuum pump was gone. Apparently its fairly common for the pump to go.

how much is "a good bit" ?

vilguy said:

how much is "a good bit" ?

Click to expand...

Hard to say, my old pump was working but just barely.
The new one when the engine is revved the hose will stick to my finger.

I was told 13.5 - 15.5 PSI
But I have no way of testing.

Thanks a lot. Ive read that its common for both to fail. Just not sure which one it is.

And like you say, hard to determine how much suction. Servo looks good too

natas said:

Hard to say, my old pump was working but just barely.
The new one when the engine is revved the hose will stick to my finger.

I was told 13.5 - 15.5 PSI
But I have no way of testing.

Click to expand...

It wont be 13.5 to 15.5 PSI as you are actually looking for a vacuum.

Absolute air pressure is about 14.7 PSI (1 Bar / 1000 mbar) at sea level, on the other hand space (an absolute vacuum) is 0 PSI (0 mbar). I would think a brake vacuum pump would produce something around 500 mbar which would read around 15 inHG if you were to use a typical boost gauge to measure it. That said it would be handy if someone with a good brake system could confirm.


Edit: just done some googling, looks like 18 inHG is a more typical minimum figure.