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  • Hydraulic Servo Laid Bare!

    Please note:- The only thing that keeps this complicated piece of kit lubricated is the PAS fluid. Incorrect or dirty fluid will lead to premature wear, a £600 bill for a new servo and about 2 hours' labour! That's assuming that failure doesn't result in brake failure... you have been warned

    Decided to see how easy this 'non serviceable item' was to disassemble... turns out that with nothing more than a pair of pliers, small screwdriver and a small punch, I took it apart in my kitchen

    Here are some pictures showing all the bits and bobs. There is only one bit I didn't take apart - the inboard shaft, which is held together with a roll pin.

    Approximate parts list
    1. Body
    2. Outboard seal
    3. Outboard shaft with hardened tip
    4. Plastic bush / cover (blue)
    5. O ring seal (black)
    6. Retaining Cap /
    7. Washer
    8. Spring
    9. O ring seal
    10. Inboard shaft (2 part) + joint + 270deg restrictor
    11. Ballbearing (possibly used as a valve or between the shaft and Retaining cap)
    12. Piston + split pin
    13. Retaining pin + seal
    14. Plastic bush / cover (blue)
    15. O ring seal (black)
    16. Retaining collar + seal
    17. Circlip
    18. Dust seal
    19. Locking nut
    20. Clevis
    21. Clevis pin






    Attached Files
    Last edited by Rusty; 3 August 2013, 19:57.
    Cheers'en, AndyC
    1994 ABY Coupe - Projekt Alpinweiss

  • #2
    A few things that I noted:-
    • The lower hydraulic fitting (where the pressure switch is) enters the bore where the ballbearing is located.
    • The upper hydraulic fitting enters at the outermost end.
    • The piston has various slots and grooves in that are controlled by the inner shaft and allow fluid / pressure to enter or escape.
    • The retaining collar is held in place, against the circlip by the hydraulic system pressure!
    • The outer roll pin holds the retaining cap against the spring, exerting pre-load on the inner shaft.
    • The inner shaft movement is limited by the retaining pin running in a slot.
    • This slot allows fluid to flow from the front chamber to the rear.

    Operation (I think!):-
    • When there is no load on the pedal, the small spring is uncompressed and the inner piston sits in its innermost postion.
    • In this state, fluid cannot flow through the servo, as the inlet slot is blocked. Anything that passes by the seals can exit via the outer chamber.
    • When the pedal is pressed, the small spring compresses and the inner piston moves outwards, covering the exit slot and sealing the end of the piston against the retaining cap.
    • This blocks the exit route for the fluid / pressure.
    • At the same time, the inner piston moving outwards opens up the inner slot and allows hydraulic pressure to pass in to the inner chamber, via the 270deg restrictor.
    • The hydraulic fluid / pressure then forces the piston outward (away from the pedal) and in turn the outer shaft, pressis on the master cylinder (the whole piston assy moves but not the retaining collar).
    • This is what helps you press the pedal outwards!
    • When you back off the pedal, the small spring is no longer under pressure, so it extends, forcing the inner piston inwards, to its normal position.
    • This closes the inner slot, stopping pressure acting on the inner chamber.
    • At the same time, the exit slot opens, allowing the pressure in the inner chamber to abate.
    • The pressure / fluid passes through a slot in the inner shaft and then out the exit hole, until both chambers are at the same pressure.
    • I think the ball bearing acts as a 1-way valve on this pathway or it could be another seal.
    • The master cylinder spring then pushes the servo open and the pedal back up.

    Attached Files
    Last edited by Rusty; 3 August 2013, 19:36.
    Cheers'en, AndyC
    1994 ABY Coupe - Projekt Alpinweiss

    Comment


    • #3
      This could be very important because, at least over here, Audi just blew the remaining stock (10 units) out on a sale. I attempted to buy one for my C4 S4 but some bashtard had bought all 10.

      Based on what I see, if the proper O-rings can be found, it looks rebuildable.

      Thanks for that Andy.
      RS2'd 93 UrS4 5 spd sedan
      94 UrS4 V8 6 spd manual avant

      Comment


      • #4
        It's a complex piece of kit for sure, there are lots of hardened seats, O rings here and there and those plastic bushes too.

        There are a few metal on metal parts but due to the lubricating properties of the PAS fluid, they should be in good condition.

        It would be essential to have any rebuilt unit pressure tested but in essence, there's nothing that I've seen that can't be replaced to take this back to nearly new.
        Cheers'en, AndyC
        1994 ABY Coupe - Projekt Alpinweiss

        Comment


        • #5
          Agreed, I am sure that any decent PAS engineer would have access to the seals and tools to rebuild that no problem.

          I'm still going vacuum though
          Panthero Coupé quattro 20vt
          Indigo ABY coupé
          Imola B6 S4 Avant

          Comment


          • #6
            More detailed pictures and part analysis to follow. The inner piston shows the worst sign of wear but is replaceable, if the part exists...

            The rest looks pretty good.

            Wear to the inner piston would prevent the 2 chambers being properly isolated and so would prevent pressure buildup, as would damaged O rings or plastic bushes. Anything that stopped the inner piston operating correctly in the larger piston would also prevent correct operation. This is the situation whee the pedal pulses and resists movement - you are pushing against a hydraulic pressure with no assistance or even negative assistance.

            The rest looks in excellent shape.

            There may be some sort of valve of the body, where the ball bearing is visible but I've not looked at taking this to bits yet.
            Cheers'en, AndyC
            1994 ABY Coupe - Projekt Alpinweiss

            Comment


            • #7
              You may (or may not) find some useful information if you search for the BMW H31 hydraulic system used on the E31 and E32 models. It's very similiar and therefore suffers the same problems so there may be some solutions too!

              Comment


              • #8
                Robin, thought you might like the title
                Cheers'en, AndyC
                1994 ABY Coupe - Projekt Alpinweiss

                Comment


                • #9
                  More detailed pictures:-

                  Outboard end of the piston with 2x seals and spring:-



                  Inner piston in inboard position (pedal not pressed) revealing exit slot:-



                  Inner piston in outboard position (pedal pushed down) closing exit slot:-



                  Movement limiting / fluid return slot on inboard shaft:-



                  270deg restrictor and bore of inboard shaft (the roll pin stops the ball bearing falling through). The restrictor also acts as a limit stop and seals on the piston inner bore but it's a metal to metal contact (as the witness marks show):-



                  270deg restrictor and inner piston (please note the spring loaded retainer in the roll pin hole through the shaft, as well as another roll pin to stop the retainer firing out in case of failure - a nice touch I thought!):-



                  Piston from the inboard side, showing inner piston bore (smaller diameter) and the hydraulic feed hole from the inlet slot:-



                  Retaining collar showing inner and outer O rings:-



                  Wear to inboard shaft (about 90deg of the diameter, opposite the slot - a design flaw IMHO):-

                  Attached Files
                  Last edited by Rusty; 3 August 2013, 19:54.
                  Cheers'en, AndyC
                  1994 ABY Coupe - Projekt Alpinweiss

                  Comment


                  • #10
                    Interesting indeed I have stripped down the earlier cast iron servo not much to it just a few o-rings to replace. Now finding those blue nylon seals might be a challenge. Really not much of anything to fail on these other than the seals, they usually end up leaking into the cabin side. The Bentley manual even states that some weeping is acceptable. Dave I guess I was one of the lucky ones and managed to get a NOS servo on discount from Audi before who ever bought up the others which sux I wish people wouldn't post the discounted items up on the forums it only encourages that to happen. Any how I think the front(engine bay side) rotary shaft seal might be the same as the one used in this servo rebuild kit.
                    http://bmw8.us/Brake%20Booster%20Rebuild.htm
                    http://denmark.esar.org.uk/car/servo/
                    Austin
                    1992 urs4 1991 200 20v avant 1990 90q 20v

                    Comment


                    • #11
                      Great thread, and links, got a problem with my S2 servo, so timely, but going to use a second hand spare one this time,

                      Thanks.

                      Comment


                      • #12
                        Does anyone know whether Audi made an uprated version of the servo which provides more assistance, possibly used by their motorsport folk?

                        I ask because I was recently shown a servo and told it was an uprated version. I checked out the part number and it looked like a standard ABY unit, so presumed the guy was talking bull. He'd bought a whole bunch of these units, so perhaps he was just trying to sell a few....
                        91 Modded 3B
                        14 A6 Avant Black Edition

                        Comment


                        • #13
                          There was the big servo used on the late LHD RS2, I'm not sure if it was uprated though?
                          Panthero Coupé quattro 20vt
                          Indigo ABY coupé
                          Imola B6 S4 Avant

                          Comment


                          • #14
                            Originally posted by UrS4boy View Post
                            This could be very important because, at least over here, Audi just blew the remaining stock (10 units) out on a sale. I attempted to buy one for my C4 S4 but some bashtard had bought all 10.

                            Based on what I see, if the proper O-rings can be found, it looks rebuildable.

                            Thanks for that Andy.
                            Dave- he paid around $75 USD each and is now seling them on ebay for over $400 each..Tidy profit!

                            http://www.ebay.com/itm/New-Audi-Bra...item3384533fd0
                            Peter S

                            1990 ErsatzS2 - track toy
                            1991 Coupe Quattro
                            1991 Coupe Quattro
                            1995 S6 Sedan
                            1995 S6 Wagon

                            Comment


                            • #15
                              Yes some rs2 LHD had the longer servo. Has the same problems as the small ones though.

                              Comment

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