Brake servo problem(?) - Audi 100 1985

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:
> "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 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.

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
: )

"Chris G." > wrote in message
. ..

> I'll post another update when the job's done.

Well, a total overhaul of the braking system seems to work wonders.

Thanks for your answers, lads; I really do appreciate your help :-)'

--
Chris
: )