Erratic idle when pumping brakes

Discussion in 'Technical' started by Hellsheep, Sep 3, 2016.

  1. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    I noticed RPM drops yesterday when in the car park and I had my foot on the brake. I then proceeded to pump the brakes to tsst it out and the car drops to 450rpm and almost stalls. If I stop pumping it comes good in about 5-7 seconds.

    I'm thinking vacuum leak as I plugged the vacuum hose at the booster side and had no issues. Is it likely the booster itself needs to be replaced or is there something I can do further to test?
     
  2. AndyZ32

    AndyZ32 Member

    Messages:
    79
    Likes Received:
    3
    Trophy Points:
    18
    . One quick test for leakage, is to turn the engine off and press the brake pedal. If the pedal still has one or two assisted applications before getting hard to press, likely no leak exists.

    Hope that helps,

    Cheers, Andy.
     
  3. Chrispy

    Chrispy Pretentious Upstart

    Messages:
    12,887
    Likes Received:
    368
    Trophy Points:
    83
    From the sounds of it your booster may be leaking. Or the hose between the booster and the check valve.

    How's your idle? Any trouble when you turn on the AC?
     
  4. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    So its about 1 full assisted brake push after the engine is off and then it's completely stiff. Also if I hold my foot on the brake pedal when the car is running if I keep putting pressure on it the pedal will go to the floor which makes me think there is something wrong with the master cylinder?

    Turning on the AC the idle drops slightly but it doesnt become erratic. It drops to about 720 from 750ish in park and idles fine when driving around and stopping on daily drives with ac on too.
     
  5. Chrispy

    Chrispy Pretentious Upstart

    Messages:
    12,887
    Likes Received:
    368
    Trophy Points:
    83
    Brake pedal shouldn't drop to the floor at any time. Something funny going on there. Bled it lately?
     
  6. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Yep bled it about two months ago when I got new wheels. I'm not dropping fluid level just checked it too.
     
  7. CHILI

    CHILI Indestructable Target

    Messages:
    9,395
    Likes Received:
    94
    Trophy Points:
    48
    Location:
    Sydney, Australia
    Sounds suspiciously like leaking/worn M/Cyl or Caliper seals(allowing fluid bypass to occur)!
    Needs to be inspected ASAP.
     
  8. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    I've got a spare Zed I can use for parts. Worth me swapping the master cylinder out myself from that one first before taking it to someone else? Seems straight forward enough to swap
     
  9. CHILI

    CHILI Indestructable Target

    Messages:
    9,395
    Likes Received:
    94
    Trophy Points:
    48
    Location:
    Sydney, Australia
    You can give that a try(but it is essential that you make sure that there is NO AIR IN THE BRAKE SYSTEM)before you start swapping-out parts/components.
    The symptoms you describe are indicative of a vacuum leak in the "Booster" system(not necessarily in the Booster itself).
     
  10. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    I guess a good way to check would be to bleed the brakes again. There was no air after the last bleed was done by me. So if there is air now it's indicative of a leak of some sort. Then from there I will bleed the brakes and see how it feels after, though the issue was present immediately already after I bled them last time and it was also present before I bled them but I didn't really think about how big of an issue it may be.

    Then I'll swap the master cylinder and possibly the booster from the other Zed which had working brakes with no issues before I started pulling it to bits. Swap them over. Bleed brakes again obviously as air will get in. Then test and see how it goes?

    Or should I just go straight for booster and mcyl swap and bleed and test? To eliminate them?
     
  11. AndyZ32

    AndyZ32 Member

    Messages:
    79
    Likes Received:
    3
    Trophy Points:
    18
    Before you rip the old components out, I'd try a little longer to pinpoint the problem, because if the replacement booster or master cylinder don't cure the fault you'll be left guessing again , especially as you'll be using second hand parts to fix it....which may themselves be then adding to the problem !
    I'd bleed the brakes again, and check to see how worn your brake pads are. I've had cars where the pads were so worn that it allowed the pistons to sit out enough in the callipers to allow air to be drawn in, without leaking. (Must admit not on the Zed though )
    Then I'd move on to your master cylinder, which sounds the likeliest culprit... after all it does most of the work. I'd invest in a new one cos the second hand one you've got is probably going to go the same way in the not too distant future given its age.
    Approaching it this way will mean you can rule out any hydraulic problem first before moving onto the booster.

    Good luck with it.
     
  12. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Thanks for that, I'll do that. Will pick up some new fluid and I do know the pads need doing so I may as well do them at the same time. I didn't know they could potentially cause that issue.

    If I need I'll pick up a new m/cyl and swap it out if the bleeding and pads don't help.
     
  13. SrAfciGeR

    SrAfciGeR Member

    Messages:
    257
    Likes Received:
    0
    Trophy Points:
    16
    Location:
    Sydney NSW, Australia

    If you have ABS which you should have, you need to bleed your brakes at certain pattern for RHD cars: RL, RR, FL, FR

    But unfortunately RPM drop it doesn't sound like that is just a brake bleeding issue...
     
  14. Chrispy

    Chrispy Pretentious Upstart

    Messages:
    12,887
    Likes Received:
    368
    Trophy Points:
    83
    And also the ABS unit in the boot.
     
  15. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Yep, I always bleed in that order for all cars, I was taught that you always bleed the furthest from the M/Cyl to the closest.

    I will be honest I actually didn't even think about this at all, nor did I believe it even had ABS. I'll need to check this, I checked the M/Cyl yesterday and it only has two hard lines from it, not three so my understanding is it has ABS. In which case I need to bleed this. Do I bleed this after I've bled the individual brakes?

    Edit:

    Found an article: http://www.ttzd.com/tech/linestech.html Will follow tonight and see how I go. Whilst I think you're right I don't think it'll resolve what appears to be some sort of vacuum leak it may at least improve things a bit especially if there is a lot of air in there!
     
    Last edited: Sep 5, 2016
  16. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Can anyone please point me in the direction of the ABS actuator? I have a 2+0 1990 NA. I have a massive sub box where the rear seats normally are in a 2+2. Please don't tell me it's there?
     
  17. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Sigh... Found it.... It's under there lol. Out comes the sub box
     
  18. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Well after bleeding the brakes and ABS unit, replacing the front pads that were badly worn, I still have the same issue. Not sure whether I should go straight to the M/Cyl here and see how I go or whether I can further diagnose this within my own capabilities/tools?
     
  19. SrAfciGeR

    SrAfciGeR Member

    Messages:
    257
    Likes Received:
    0
    Trophy Points:
    16
    Location:
    Sydney NSW, Australia
    As suspected your RPM drop doesn't have anything to do with the bleeding.
    But now you have descent breaking system!

    Let's get back to the vacuum system which is most likely to cause issues you experiencing.

    If you can isolate brake booster "be creative when plugging something in the vacuum line that goes to brake booster, so that you can take it out" and then try pumping your brakes...

    If you still have same issues problem might be electrical.

    If problem goes away, well you will have to change brake booster...
     
  20. Hellsheep

    Hellsheep Administrator Staff Member

    Messages:
    244
    Likes Received:
    32
    Trophy Points:
    28
    Did this already before I created the thread. I pulled the vacuum line from the booster and plugged it tight. Issue does not occur when this line is plugged.

    Could it be something else downstream of the booster however and not the booster itself?
     

Share This Page