Conclusion

[JIM_WARREN@HP-FtCollins-om4.om.hp.com]

Item Subject: cc:Mail Text
     Hi guys, I'd like to test my conclusions on my brake problems.
     Car is 1992 S4.
     
     Symptoms:
     
     *  Brake boost unavailable at start up about 50% of the time
     *  Brake light stays on for 30-60 seconds after releasing e-brake
        after car has sit for more than 1 hour  (This symptom has existed
        for several years)
     *  Sometimes after sitting, brake pedal depresses in jerky motions,
        instead of smooth travel.  Smooth travel returns after hydraulic
        pressure builds up.
     
     Diagnostics:
     
     *  Pentosin and brake fluid levels OK.
     *  Bomb OK (passes bomb test consistently)
     *  I cracked open low-pressure (return) line from brake
        servo.  Lots of fluid comes out when engine idling.
        (A lister told me this should be only a few drops for
        a good servo.)
     *  Pulled master cylinder from front of servo.  Everything looked
        clean and normal inside servo housing.  No leaking fluid.  (Not
        sure exactly what I was looking for.)
     
     Conclusion:
     
     *  Bad servo.  Allows hydraulic pressure stored by bomb to leak 
        through servo.  When car is started, it takes a while for full
        hydraulic pressure to build up and operate the inlet pressure
        switch on the servo, which is connect to the brake light.  Perhaps
        some pressure even needs to build up in the return line before the
        switch is triggered.  This could also explain why I have never been
        very happy with the brake pedal feel in this car.  I loved the
        pedal feel in my old 87 5000CSTQ (very solid), and was surprise
        when I bought the S4 (used) in the difference.
     
     Repair:
     
     *  It looks easy.  Two bolts to remove MC.  Disconnect brake pedal
        linkage.  Four nuts (inside of firewall) to remove servo.  R&R.
        Anyone BTDT that can warn me of any gotchas?  Does this diagnosis
        sound correct?
     
     Thanks,
     
     Jim Warren
     1992 S4
     Fort Collins, CO