[s-cars] Front Brake calipers not releasing.....Master Cylinder?

[Scott Justusson]

Been reading this thread with interest, and plenty of btdt on S cars.  A
couple of items to note.  First, there are two master cylinders for the
S car, one for the 3 channel ABS (92>95) and one for the 4 channel ABS
(95.5>97).  I haven't read it, but it appears this is an early?  Second,
neither master cylinder is considered rebuildable.  The circlip used is
internal with tapered (tamperproof) tabs.  That said, I've rebuilt a
couple late style Master Cylinders, but a cut is required in the lip of
the master to access the circlip.  It's not a fun procedure, as there is
a bunch of exactly mounted hardware in that bore.

Both masters use discs and antireversion dampening in the fluid return
circuit.  These are very small passages that serve to slow down and
dampen fluid return into the master cylinder.  IME, they can get clogged
easily, especially with old fluid, or running ATE Superblue Fluid.  IME,
most Master problems on S cars are caused by this symptom.  That said, I
have also seen bad brake line internals cause the exact same problem
too, as Scott N notes this am.

It is possible to get a clogged master to come back to life, btdt.  It's
a messy job with compressed air, a brake bleeder and lots of brake
fluid.  Flush and back-flush the master with a liberal amount of brake
fluid (speaking by the quart here), to clean out the passages.  IME2, I
do this job with the additional condition of replacing the 4 rubber
lines to the respective calipers.  Even slightly collapsed lines can
cause one to chase the master when it's not.

I'm surprised this is something 'new' on any S car forum, as I have run
into this problem routinely on the S car in the last 10 years.

HTH and my .02

Scott J


-----Original Message-----
From: s-car-list-bounces at audifans.com
[mailto:s-car-list-bounces at audifans.com] On Behalf Of Tom Green
Sent: Wednesday, April 22, 2009 7:16 AM
To: Scarlist; Jim Fleischer
Subject: Re: [s-cars] Front Brake calipers not releasing.....Master
Cylinder?

>
All symptoms point to the master cylinder, Jim.

You did not mention how you were bleeding the brakes.  Do you have a
power bleeder now?  If so, what kind?

I believe you are experienced at brake work, and fully understand this
question,but have you verified that the correct fluid is in each system?

Tom

> -----Original Message-----
> Date: Tue, 21 Apr 2009 19:45:27 -0600
> From: "Jim Fleischer" <jim at almgt.com>
> Subject: Re: [s-cars] Front Brake calipers not releasing.....Master
>   Cylinder?
> To: "Lee Levitt" <lee at wheelman.com>
> Cc: s-car-list at audifans.com
> Message-ID: <DFA9E02A978D4817AE02080AE70742A0 at 421Capitol>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
>   reply-type=response
>
> The part #'s matched up front and rear, and the shape of the pads were
> identical to what I took off.......they fit fine until the first  
> brake pedal
> test.....then they just stay there.  I took the car out again after  
> bleeding
> the whole system, twice......after a very short drive, I checked all  
> the
> corners and it looks like I've got issues at all four calipers, as  
> all the
> rotors were hot......the fronts are just making more noise than the
> rears.....hopefully the master cylinder will do the trick.
>
> thanks for the tip, though....
>
> Jim

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