[s-cars] Brakes-S6, soft and spongy.. LONG

Sun Apr 23 09:03:25 EDT 2006

Fred,

I was hoping YOU would write, thank you.   I plan to right a Munro style
writeup to document my experience.
Certainly won't drive it Fred!

"You should not have had to open the rear bleeder to retract the caliper
pistons. The fluid should move easily back into the MC reservoir when
you retract the pistons."

Agreed, this was the weird part although the only explanation I can
think of is insufficient pressure on my hand operated piston retractor
tool. For the record I used pressure bleeding method, although many
experienced people I respect wrote back recommending 'vacuum bleed' at
each caliper as the best of all.

"Bad Bomb....the pedal vibration is resulting from a discharged bomb."
That's a valuable explanation!  Bomb discharge is at least one failure
mode. Given the dash brake light stays on (even after 15 minutes) and
the 2 pumps to hard pedal, I read the Bentley about testing hydraulic
pressure.  
I do not have a gauge suitable for 2000 psi, so I improvised.

In this particular case ( and only this)  I decided to wrap a rag around
the high pressure line at the Brake servo input, (don safety goggles)
and crack open that line slightly. Normally this is not recommended as
fluid under high pressure would squirt out of there. But in this case, I
expected nothing to happen assuming the Bomb is dead. Turns out no fluid
came out of this line at all. Bentley has a decent trouble shoot
procedure for those who want to make a 2000 PSI gauge and steel line
gizmo.

It's either the check valve IN the Bomb, and/or the lack of Nitrogen
pressure. In any event I am buying a Brake Accumulator tomorrow. I
removed the Bomb and found it to weigh about 3 pounds, maybe it's full
of Pentosin!
IT's no obvious how to service this at all. 

Rear Pad Clearance- Good point. Peter Schulz recommended I cycle the
parking brake several times to ratchet out the pistons in the rear. This
done, I think this item is closed now.

"You don't have to discharge the bomb when pressure bleeding the brakes.
The brake system and the hydraulic boost system are completely separate,
connected only by the mechanical action of the servo on the MC."

Exactly Fred, it took me a little while to draw a correct hydraulic
schematic and understand the purpose of the brake servo. I'll include
this in my writeup. 

Thanks to ALL you guys who wrote on S car list and private email. I now
fully understand this well enough to fix my brakes.

Jeff

-----Original Message-----
From: Fred Munro [mailto:munrof at sympatico.ca] 
Sent: Saturday, April 22, 2006 6:50 PM
To: Postupack, Jeff; s-car-list at audifans.com
Subject: RE: [s-cars] Brakes-S6, soft and spongy.. LONG

Hi Jeff;

Sounds like you have two problems; a bad bomb/pump and too much
clearance in the rear pads.

Bad Bomb: The pedal vibration is caused by the pressure pulses from the
pump. This is only felt when the bomb is discharged. The same with the
pedal slowly moving forward as the car runs - with the bomb discharged,
the pedal is high and hard. As the bomb charges and you get power
assist, the pedal stroke increases and the pedal gets "soft". If your
bomb only holds 2 brake applications, either the bomb is dead or the
hydraulic pump isn't putting out enough pressure or volume to charge it.
I either case, the car is unsafe to drive.

Rear Pad Clearance: The UrS brake systems are quite sensitive to pad
clearance. Too much clearance results in a low pedal. The e-brake
mechanism ratchets out the rear pads to keep the clearance tight. After
a pad change the clearance is usually too large. You have to bed in the
pads and use the e-brake to tighten up the clearance and restore a high
pedal. The same effect (low pedal) results from seized caliper carrier
pins or pads rusted into the carriers as both conditions don't allow the
pads to move into close proximity to the rotor face.

As for air in the system, it you used a pressure bleeder and didn't
allow the MC reservoir to go empty, it is unlikely you introduced air
into the system. The MC is usually bench bled by making up short lines
that run back into the top of the reservoir and stroking the MC until no
air comes out of the lines. Once on the car you shouldn't have to bleed
the MC unless you empty the system for some reason.

You should not have had to open the rear bleeder to retract the caliper
pistons. The fluid should move easily back into the MC reservoir when
you retract the pistons.

You don't have to discharge the bomb when pressure bleeding the brakes.
The brake system and the hydraulic boost system are completely separate,
connected only by the mechanical action of the servo on the MC.

It sounds to me like your soft pedal is resulting from excessive rear
pad clearance and the pedal vibration is resulting from a discharged
bomb.

HTH

Fred Munro
'94 S4
'97 S6

-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com]On Behalf Of Postupack, Jeff
Sent: April 21, 2006 11:50 AM
To: s-car-list at audifans.com
Subject: [s-cars] Brakes-S6, soft and spongy.. LONG

Awright!  I need help, have to break down and seek S List counsel.

Situation is 1995.5 S6 Brakes.

Problem= spongy brakes, the pedal vibrates (lightly) when depressed, the
brakes are not firm.
Can't leave the property..

Observations:
1-Brake pedal was firm before I started mucking around!
Why did I fix it?  I'll never answer that question!~ frankly needed to
replace rear pads only.
2- I did NOT de-pressurize the bomb at the start, so I do not know the
start condition of the Bomb.
I 'later' read depressurizing the Bomb is advised.

3- Front brakes are OEM, in fact this entire S6 is Stock.

Changed the REAR pads only, used a caliper piston compressor to open the
gap, and reinstall.
I had to open the rear bleeder to move (open)  the pistons and drain
some brake fluid.

Bleeding: Built a pressure bleeder (thanks to Huw Powell, Tom Mullane)
and bled all 4 corners.

I searched the S car list archives, Audiworld, S-cars.org , www.20V.org,
and to my surprise I did not see a definitive brake bleed procedure for
the
C4 chassis specifically. A few writeups mentioned techniques for ABS
equipped Audi's , tried to follow those ideas closely.

Yes, I read the Bentley,[ I own the paper manual set] not much there
frankly..

 but I gotta tell you, the bleeding procedures by Phil Payne and 20V.org
are much more comprehensive.

More Observations
1- The 1995.5 S6 does NOT have a brake proportioning valve located on
the Left side rear wheel area.
(My 1993 S4 DOES have this valve in the rear)

2- My 1995.5 S6 does not have a bleeder at the Master Cylinder. (How do
I clear the air from the MC?)

Method:

A: applied Super Blue with my Pressure bleeder at 10 PSI, Andrew made
sure no air bubbles entered the MC under pressure.

B: I bled the Right Rear, Left Rear, Right Front, Left Front calipers ,
the clutch slave cylinder first, in that order.

C: I started the engine and pressed the brake pedal, observed the pedal
'vibrated' slightly and then gradually moved forward, never reaching the
firm pedal I noticed when I  STARTED this whole process.

D: NOW I think to test the BOMB, shut off the engine and tested number
of pedal depresses.
Counted 2, before the pedal got rock hard!

E: I pressure bled the system once again, more Super blue flowed into
the spend tank!

F: I started the engine and pressed the brake pedal, observed the pedal
'vibrated' slightly and then gradually moved forward, never reaching the
firm pedal I noticed when I  STARTED this whole process.

Same results. WTF!

I realize it is tough to diagnose via email, Is that BOMB dead?  It
COULD have been on the verge of failure given 10 year life.
Is it possible I STILL have air in the system?

Whattaya think?

Posto

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