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
: )



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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)

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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
: )



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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)

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Chris G. wrote:

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
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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)

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Steve Sears wrote:

Agreed
Yes both options are possible.

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.
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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/showthread.php?t 01029134&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)

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Steve Sears wrote:

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

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

Seems like a reasonable assumption that the master cylinder is ok. Good luck with your project!
Regards
Wolfgang
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It seems to me that there is another explanation.
If the Brake fluid is low then it would still stop. If it is gone from the MC it wouldn't stop and the pedal would go to the floor.
A sudden change in brake pressure (pedal is hard to press but car stops) seems more like a failure in hydraulic assist > the pump is not supplying assist to the MC. This could be due to the Pentosin being low / gone, the pump failing or the belt that drives the pump breaking.
The 'bomb' would not seem to cause this as it is used to provide assist when the engine is off. If the bomb is bad but the pump is working it will stop fine but throw a warning light on hard braking.
The problem with my theory is that this failure would also eliminate the power steering but his girl friend didn't mention that.
My bet is the hydraulic fluid (Pentosin) is very low or the pump belt is gone.
After loosing a pump on a trip I drove 500 miles + with no assist. Its a bit scary but it works.
TonyJ previously four T44s current 98.5 A4 1.8 TQM
Chris G. wrote:

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Well, it looks like two new rear calipers are in order here, with all the associated pipework :-)
It's been quite a hard, cold winter here in Denmark, and it seems as though the road-salt has done it's evil work seizing some things up and eating through the rest :-/ Everthing else with the braking system functions perfectly (thankfully).
I'll post another update when the job's done.
--
Chris
: )



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Well, a total overhaul of the braking system seems to work wonders.
Thanks for your answers, lads; I really do appreciate your help :-)'
--
Chris
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