Interesting Brake Servo question

Wild guess, since i'm not a mechanic...

Don't brake servos operate via vacuum, so if you're letting the car roll back "naturally", there's very little in the way of vacuum generated by the revving of the engine ?

You have maximum vacuum at idle (on a petrol, other than at over-run)

- but im fairly sure it is vacuum related.

"Colin Wilson" wrote in message news: snipped-for-privacy@news.individual.net...

Maximum vacuum is created at idle speed, less the harder the loud pedal is pushed.

If servo assistance disappears instantly (as opposed to having a few normal presses of the pedal) then there must be a massive vacuum leak or something jamming the servo (such as a build-up of fluid in the servo)

Unless of course there is actually something jamming the brake pedal mechanism

Mrcheerful

Underused back brakes that simply don't work, and a biasing valve that's sending most of the fluid to the back?

Nope, back brakes seem fine, discs are nicely "cleaned" after a few stops and do get warm after a bit of use so I assume theyre working ok. The change is pretty instantaneous though so i see your thinking and you might be right, though the pedal firms up like when you cut the engine so I had assumed it was vacuum based.

On 9 Aug, 23:54, "mrcheerful ."

I think it must be a leak but the hoses all look fine after a fumble with them, it really is as though the servo assist is just cut - the pedal firms up just like when you cut the engine and press until you run out of vacuum, only faster. I just cant find anything that would leak only when reversing/going backwards downhill.

Nope, checked that one out, but it is the sort of easily overlooked thing I would miss!

I once had to work on a Vauxhall cavalier with an odd misfire, it turned out to be the servo pipe, it was cracked (hard plastic at the end) and when the engine twisted a bit it leaked

I'll go back over all of the hoses and remove and re-install them all to be sure.

I'm not sure I'm firing on all cylinders with this one, but wouldn't the engine rock the opposite way to normal in reaction to putting down power in reverse?

Love to hear the eventual solution...

Yes, if the engine rocked much, though it is pretty solid - blipping the throttle doesnt move it and you really can balance a 50p on the engine while its running lol.