Brake servo problem(?) - Audi 100 1985

Hey (again :-) group

Well, aircon's on the back burner again due to the following:

My girlfriend was on the way to work this morning when the brake-servo
seemed to give up the ghost - there was a total loss of brake pressure, in
any case. I've taken a look at the Haynes manual, but was wondering if
there's anyone here that can point me in the direction of the most likely
failure? The warning light was on (along with three beeps) when the failure
occurred.

Thanks for any help you can offer.

--
Chris
: )

Chris,
Does "total loss of brake pressure" mean that the pedal went to the floor,
or does it mean that you need superhuman strength to push on the brake pedal
to make the car stop? If the pedal went to the floor, chances are you
either have a leaking brake line, or the master cylinder has failed. The
other failure can be due to the failure of the "bomb" - hydraulic pressure
accumulator. There's a test on www.sjmautotechnik.com for it - the
accumulator lists for big bucks at the "usual suspects", but there is a guy
on the Audifans quattro list that is retrofitting accumulators to make them
rechargeable at any heavy equipment shop (that can supply 2000 psi
nitrogen) - I have one on my car (different guy, but same forum) and it has
worked well, but is in need of a recharge (they all lose pressure over
time).
Cheers!
Steve Sears
1987 Audi 5kTQ
1980 Audi 5k
1962 and '64 Auto Union DKW Junior deLuxes
(SPAM Blocker NOTE: Remove SHOES to reply)


"Chris G." > wrote in message
. ..
> Hey (again :-) group
>
> Well, aircon's on the back burner again due to the following:
>
> My girlfriend was on the way to work this morning when the brake-servo
> seemed to give up the ghost - there was a total loss of brake pressure, in
> any case. I've taken a look at the Haynes manual, but was wondering if
> there's anyone here that can point me in the direction of the most likely
> failure? The warning light was on (along with three beeps) when the
> failure occurred.
>
> Thanks for any help you can offer.
>
> --
> Chris
> : )
>

Chris,
You may find that it's the flex line. You should replace all 4 flex lines
instead of just the one (they're all due). Be careful when disconnecting
the hard lines, if they're as stubborn as the ones on my car you'll need to
replace some hard line as well (luckily the local FLAPS has metric bubble
flared brake lines in various lengths). Don't go for the "bling" factor
stainless steel braided lines - the rubber lines are the way to go.
Cheers!
Steve Sears
1987 Audi 5kTQ
1980 Audi 5k
1962 and '64 Auto Union DKW Junior deLuxes
(SPAM Blocker NOTE: Remove SHOES to reply)

"Chris G." > wrote in message
. ..
> Hey Steve - thanks for the quick reply It seems that there's a leak in
> the region of the rear-left caliper - oil mist on the inner-side of the
> tyre, a small drip-pool where the car's been parked up today. Doesn't seem
> like it should be a prob to sort, and cheaper in any case than replacing
> the pressure accumulator; every cloud has it's silver lining ;-)
>
> --
> Chris
> : )
>

Chris G. wrote:

> Hey Steve - thanks for the quick reply It seems that there's a leak in
> the region of the rear-left caliper - oil mist on the inner-side of the
> tyre, a small drip-pool where the car's been parked up today. Doesn't seem
> like it should be a prob to sort, and cheaper in any case than replacing the
> pressure accumulator; every cloud has it's silver lining ;-)

IMO there's a second problem here.

That car should have two separated braking circuits (i.e. left front
operating together with right rear and left rear with right front).

Both systems are actuated via one master cylinder which also separates
the systems.

This should make sure, that even if one braking line blows, the other
should still be working so that you can still - although with higher
foot pressure - bring the car to a stop with only two brakes in operation.

If that car really lost it's brake completely there must be a leak
between circuit 1 and 2, because only in this case the pressure in the
second 'good' circuit could bleed away as well.

Most likely the seal in the master cylinder between the two circuits is
leaking. So you should check that too.

With the loss of one braking line such a car must never lose all its
braking power.

BTDT in a Ford.

Regards

Wolfgang

Wolfgang,
If the loss of fluid was enough that the master was sucking air, then it may
have felt like a total failure of the brakes....maybe? I agree that there
may be problems with the master cylinder - both from what you note about the
dual diagonal brake circuits - and if the brake fluid was not changed
regularly (~ every 2 years or so), the portion of the cylinder bore that
never saw the piston seals may have developed some corrosion that could
damage the seals when the brake pedal was pressed to the floor. I would
make sure the brakes are really well bled after hose replacement, and then
hold the car on the brakes to see if the pedal creeps down. As brakes are
nothing to be trifled with...<insert standard disclaimers here>
Cheers!
Steve Sears
1987 Audi 5kTQ
1980 Audi 5k
1962 and '64 Auto Union DKW Junior deLuxes
(SPAM Blocker NOTE: Remove SHOES to reply)

"Wolfgang Pawlinetz" > wrote in message
news:1143558183.457287@news...
> IMO there's a second problem here.
>
> That car should have two separated braking circuits (i.e. left front
> operating together with right rear and left rear with right front).
>
> Both systems are actuated via one master cylinder which also separates the
> systems.
>
> This should make sure, that even if one braking line blows, the other
> should still be working so that you can still - although with higher foot
> pressure - bring the car to a stop with only two brakes in operation.
>
> If that car really lost it's brake completely there must be a leak between
> circuit 1 and 2, because only in this case the pressure in the second
> 'good' circuit could bleed away as well.
>
> Most likely the seal in the master cylinder between the two circuits is
> leaking. So you should check that too.
>
> With the loss of one braking line such a car must never lose all its
> braking power.
>
> BTDT in a Ford.
>
> Regards
>
> Wolfgang

Steve Sears wrote:
> Wolfgang,
> If the loss of fluid was enough that the master was sucking air, then it may
> have felt like a total failure of the brakes....maybe?

Agreed

> I agree that there
> may be problems with the master cylinder - both from what you note about the
> dual diagonal brake circuits - and if the brake fluid was not changed
> regularly (~ every 2 years or so), the portion of the cylinder bore that
> never saw the piston seals may have developed some corrosion that could
> damage the seals when the brake pedal was pressed to the floor.

Yes both options are possible.


> I would
> make sure the brakes are really well bled after hose replacement, and then
> hold the car on the brakes to see if the pedal creeps down. As brakes are
> nothing to be trifled with...<insert standard disclaimers here>

Yep. I would just try to check if the circuits are still separated. That
would make Me nervous.

A simple check would be to bleed off the air in one circuit, leaving the
other one open. Then apply foot pressure. The pedal still shouldn't
creep to the floor, right?

Regards

W.

Wolfgang,
I'm almost positive that when I last bled the dual-diagonal circuit brakes
on both my 5ktq and 5k manually (I use a home-made pressure bleeder now), I
could push the pedal to the floor when I had my friend crack the bleed screw
on one corner only. I checked on the net and there are warnings about not
pressing the pedal all the way to the floor:
http://forums.probetalk.com/showthre...1029134&page=1
(third post)
Maybe Chris' master cylinder is not malfunctioning......maybe?
Cheers!
Steve Sears
1987 Audi 5kTQ
1980 Audi 5k
1962 and '64 Auto Union DKW Junior deLuxes
(SPAM Blocker NOTE: Remove SHOES to reply)

"Wolfgang Pawlinetz" > wrote in message
news:1143560984.347220@news...
>
> Yep. I would just try to check if the circuits are still separated. That
> would make Me nervous.
>
> A simple check would be to bleed off the air in one circuit, leaving the
> other one open. Then apply foot pressure. The pedal still shouldn't creep
> to the floor, right?
>
> Regards
>
> W.

"Wolfgang Pawlinetz" > wrote in message
news:1143558183.457287@news...
> IMO there's a second problem here.
>
> That car should have two separated braking circuits (i.e. left front
> operating together with right rear and left rear with right front).
>
> Both systems are actuated via one master cylinder which also separates the
> systems.
>
> This should make sure, that even if one braking line blows, the other
> should still be working so that you can still - although with higher foot
> pressure - bring the car to a stop with only two brakes in operation.
>
> If that car really lost it's brake completely there must be a leak between
> circuit 1 and 2, because only in this case the pressure in the second
> 'good' circuit could bleed away as well.
>
> Most likely the seal in the master cylinder between the two circuits is
> leaking. So you should check that too.
>
> With the loss of one braking line such a car must never lose all its
> braking power.

Yep, I can see the point, problem being that I didn't experience the failure
myself - my girlfriend said that the brakes *worked*, but that they needed
more strength than she could muster....

I can see from the pile of workshop-receipts from the previous owner that my
idea that the master cylinder was correct; he'd had it renewed at an Audi
dealer last november.

Well, I'll locate the source of the fluid mist on the rear tyre (it is
certainly from around the caliper somewhere) and see how that goes.

Steve: agreed with the braided lines; I use them on my motorcycle, but for a
car I've always favoured OE rubber lines - it's longevity I'm after on the
car, not brake feel

I'll come back with more when my bout of 'flu is over and I've had a chance
to deal with this.

--
Chris
: )

Steve Sears wrote:
> Wolfgang,
> I'm almost positive that when I last bled the dual-diagonal circuit brakes
> on both my 5ktq and 5k manually (I use a home-made pressure bleeder now), I
> could push the pedal to the floor when I had my friend crack the bleed screw
> on one corner only. I checked on the net and there are warnings about not
> pressing the pedal all the way to the floor:
> http://forums.probetalk.com/showthre...1029134&page=1
> (third post)
> Maybe Chris' master cylinder is not malfunctioning......maybe?

Maybe. I'm just wondering what sense it then makes to have a diagonal
separated system anyway.

Honestly I am pretty sure the pedal should not creep with one line
broken. At least that was told to me by my previous Ford Mech. The
principle is the same.

I'll try to check with my Audi garage in the next days.

Regards

Wolfgang

On Tue, 28 Mar 2006 19:10:29 +0200, "Chris G."
> wrote:

>Yep, I can see the point, problem being that I didn't experience the failure
>myself - my girlfriend said that the brakes *worked*, but that they needed
>more strength than she could muster....

Ok. That sounds as if the spare circuit worked. :-)

>I can see from the pile of workshop-receipts from the previous owner that my
>idea that the master cylinder was correct; he'd had it renewed at an Audi
>dealer last november.
>

Seems like a reasonable assumption that the master cylinder is ok.
Good luck with your project!

Regards

Wolfgang