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

[Fred Munro]

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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