Interesting Brake Servo question

Well I think its interesting. '90 BMW 535i. Brakes work just fine unless backing the car down the path then all servo assist seems to vanish (mildly scary on an automatic on a steep path). Checked the fluid levels, all good. Brakes are rock solid (as you expect from a bimmer) so no bleeding issues etc, it just seems to be when its on a slope. Get your teeth into that?!

Reply to
CoyoteBoy
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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 ?

Reply to
Colin Wilson

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

- but im fairly sure it is vacuum related.

Reply to
CoyoteBoy

"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

Reply to
mrcheerful

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

Reply to
Doki

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.

Reply to
CoyoteBoy

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!

Reply to
CoyoteBoy

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

Reply to
Mrcheerful

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

Reply to
CoyoteBoy

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?

Reply to
Doki

Love to hear the eventual solution...

Reply to
Dave Plowman (News)

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.

Reply to
CoyoteBoy

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