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Sticky annoying servo

Discussion in 'Brakes' started by heapy, Mar 29, 2017.

  1. heapy

    heapy Member

    Hello, i noticed my original lockheed servo was leaking, so instead of repairing i thought id just replace. i went for an uprated "Powertune" replacement off ebay.
    Am i missing something or are these just rubbish? i've bled and bled it through the old fashioned 2 people method, i've even tried releasing the unions on the servo too but it's always the same. The breaks whilst not spongy seem to stick when pressing the pedal. poor braking until you really press hard then you lurch forwards from sharp braking. if you give them a little pump then re apply they work quite well. you can also feel the brakes just momentarily sticking on for a second after you release your foot. i've been through the calipers, they're fine and every rubber pipe has been replaced too. This has all only happened since the servo change.
    I had the same story years ago with another standard Powertune servo. so i'm thinking it must be me!!
    Car is a 1970 Mk1 P6 2000 sc auto.
    Many thanks.
  2. harveyp6

    harveyp6 Well-Known Member

    I think you have your answer there.
  3. sdibbers

    sdibbers Well-Known Member

    Hi Heapy,

    Brakes sticking suggest the airvalve is a little sticky. I had this problem after I rebuilt my servos. This can fix that problem:
    1) Pull cap off flying saucer shaped valve, make sure you catch the filtre and spring as you do. (Spring should stay with the cap)
    2) Pull spring off cap and stretch it slightly.
    3) Replace spring, filtre and cap onto air valve.
    4) Test drive

    As for the second stroke to get good brake feel I would suspect that one of the rear calipers isn't self adjusting correctly. You can test that relatively quickly by chocking the front wheels, jacking up the back end and sliding under the car. Make sure the handbrake is fully off. Give both calipers a good wiggle, there should be a very small amount of movement detectable, if one feels looser than the other I would suspect it's adjuster needs some help.

    Good luck,

  4. heapy

    heapy Member

    Thanks to you both! i'll have a play with the servo and then check the rear calipers of death! Not a fan of going near them as my axle stands and jack only lift the car enough for the de dion to be low enough to be accessible ( just enough so it sits across touching my neck with a ton and a half above ! ha! ) wasn't the most pleasurable experience when i bled them last time. However, time to replace the pads very soon as they're low and probably from 1975 due to the low miles! So maybe an all in one job.
    I have the original servo still, so if it's still annoying after, i'll just replace the seals after a quick hone possibly. Just wanted to make sure there wasn't some strange way of bleeding it! The Powertune servo wasn't cheap either :/
    Cheers, Keith.
  5. sdibbers

    sdibbers Well-Known Member

    It might be that the piston in the air valve is just sticky from being new. You don't know how long it was sitting on a shelf before being sent to you.

    Before you go under the back end how does the handbrake feel? If it goes a long way up before biting that can also indicate the caliper problem.
  6. clive P62

    clive P62 Active Member

    That little piston is renown for sticking.
    In the kits I have used it now comes with a sachet of silicon grease to use on the air valve piston and main piston plastic bearing.
    sdibbers likes this.
  7. Tony58

    Tony58 Member

  8. Tony58

    Tony58 Member

    Hi just read your post about a sticking servo, mines the same with the brake lights and brakes stuck on until I take the little vacuum pipe off the servo to air vent, would you replace the inards or would a strip clean rebuild suffice tony
  9. cobraboy

    cobraboy Well-Known Member

    I'll stick my neck out and say that if you have a problem with an original servo then it is time it was either rebuilt or exchanged. If you open it then it will need at least seals and will probably need re sleeving as well. Don't put it back together with just a clean up of the parts, it will be a waste of your time.
  10. Tony58

    Tony58 Member

  11. Tony58

    Tony58 Member

    Yes that's my intention, sourcing the bits as we speak
  12. classicalgreen

    classicalgreen Active Member

    got an exchange servo and all week until just after MOT test ( passed hurrah) then brakes stuck on all the way home. ( I thought it was a fuel supply problem as get a lot of that) having simply released pressure in lien form servo. brakes now function. OK though very slight drop in. pedal position due to small amount air in pipes .( will bleed. again later)
    checking on various forums it become apparent theres a small 'valve' and spring in valve assy. above servo that has a rep for 'sticking' especially when. not used for long periods!
    moisture ? corrosion? who knows but no further issues and your initial problem. sounds exactly like mine .a sticking. valve.
  13. Tony58

    Tony58 Member

    Thanks I've stripped that valve, can't see what it does, if you blow into the valve even when you've pushed it in still allows air in, anyway cleaned it put it back on the servo and the brakes returned to normal, wait and see, was going to buy a new one, being a 9" thought about powertune but people seem to have problems with that make
  14. cobraboy

    cobraboy Well-Known Member

    Ah yeah but...………. don't just buy any 9" it has to have the right boost ratio, 4.25 :1 for a V8 car ( if that is what you have )
  15. Tony58

    Tony58 Member

    That was my next question, the boost ratio, I've found ang do a full rebuild kit, plus the air valve, no change out of £100 tho, might start with a new air valve first and see how I go on, don't know how you split it to get to the big diaphragm there's no circular clip on this
  16. ButterFingers

    ButterFingers Active Member

    In the workshop manual it shows a special tool for unscrewing, anticlockwise, the rear of the servo. its something that could be made, if clever with a mig and bits of metal.
  17. sdibbers

    sdibbers Well-Known Member

    Yep, I made one from two bits of angle iron and some square tube. Pretty easy to make. You will need a bench vise too
    ButterFingers likes this.
  18. Vern Klukas

    Vern Klukas Active Member

    A monster screwdriver or pry bar will work too, with a little care. The vise is essential though.


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