Locking brakes again | The Z Club of Great Britain
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Locking brakes again

Discussion in 'S30 (240Z,260Z,280Z) General' started by Jamesk350, May 6, 2019.

  1. Jamesk350

    Jamesk350
    Z Club Member

    Hi all I’m looking for some help/advice.
    I’ve got a 76 280z, after about a half hours drive which is around 8 to 10 miles the brakes on the car are starting to be binding and short after that the brakes are locked on and the car can not be pushed by hand.

    The car has a 280zx servo and master cylinder added, it’s had the plunger set at FourWays and new Toyota front brake callipers and pads fitted. Previously to that the brakes got really hot when binding/locking up now they still seize but don’t get hot at all.
    Lastly I have replaced the left rear wheel cylinder as it was noted when the last time it was seized only the right rear wheel was spinning (it’s was at a car show on grass) but while the overheating issue has gone the car still seized up. It’s worth noting that after the is left it free wheels with little effort.

    Any ideas or anyone had this or similar issues?
     
  2. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    Which wheel(s) are locking up?
    Sound like most or all, which would suggest the master cylinder isn't releasing when you take your foot of the brakes.
    Either the rods not adjusted right or maybe the servo is acting when you're not pressing the pedal (I don't understand the system well enough to know if that's possible!) easy enough to disconnect the vacuum line and plug it and test.
    Did fourways set the master cylinder up? I'd expect them to sort it if that's where the problem is.

    BTW, if the problem is at the master cylinder and the right rear was still turning, that would suggest the right rear cylinder has a problem. It should have locked up too.
     
  3. Jamesk350

    Jamesk350
    Z Club Member

    Jon.
    All wheels seem locked on even with the handbrake off. Just thought now you mentioned it the engine drops from it’s idle to about 500rpm and started to cut out.
    After I’d left the car for 20mind the car rolled freely again but still cut out.
     
  4. Farmer42

    Farmer42
    Z Club Member

    It sounds as though your servo is not releasing the vacuum pressure after you apply the brakes. It could be a faulty servo or a faulty pressure valve in your vacuum pipe. Run your car for a while with your foot on the brake and if it locks on disconnect the vacuum pipe from the servo. It should release the pressure and thus release the brakes. If not, the servo has a fault.
     
  5. jonbills

    jonbills
    ZClub Administrator
    Staff Member Moderator

    Sounds like it might be worth disconnecting and plugging the servo vacuum line at the engine/manifold/plenum then.
     
  6. toopy

    toopy
    Z Club Member

    How does the pedal feel when the brakes are locked on, with the handbrake off, same as usual, softer/harder?

    If it's the same as when the brakes are working normally, that would imply to me that the vacuum booster is operating normally and it's the MC that's at fault.

    Brake pressure regulator valve?
     
  7. atomman

    atomman
    Z Club Member

    have you replaced all the flexi's ? they may look ok but if they have collapsed inside they will let the fluid through when you press the pedal but wont let it flow back , hence locking the wheel when the fluid gets hot and expands,
     
  8. Jamesk350

    Jamesk350
    Z Club Member

    The brake pedal feels the same all the time, it does not change when the brake lock on. I’ll have to check the vacuum next time I’m out with it.
     
  9. Jamesk350

    Jamesk350
    Z Club Member

    Front pipes have been replaced 6months ago. The rear ones where replaced about 4 years ago by the last owner
     
  10. stuart barrie

    stuart barrie
    Z Club Member

    Just checked setup in Haynes manual page 135
    Seems bit weird way to setup pedal for but this is what it says.
    Loosen lock nut on the pedal arm stop bolt and screw the stop bolt fully in.
    Release the lock nut on the servo unit pushrod and rotate the pushrod until the height of the upper surface of the brake pedal pad is 206mm above the metal surface of the toeboard . Lock the pushed nut.
    Unscrew the stop bolt until the pedal height is reduced to 203mm. Lock the stop bolt nut. The pedal travel should be 140mm.
    These measurements will be all off if the floors have been replaced?
    Any I have done in the past there is normally a measurement of free play before the master cylinder is activated allowing pressure in system to release.
     
  11. toopy

    toopy
    Z Club Member

    That's only for adjusting pedal height, the push rod on the back of the servo that connects to the pedal arm should not be under any tension with the pedal at rest.
    The screw adjustment is purely for setting the pushrod length relative to the pedal once it is it's self properly adjusted and not for adjusting the braking effect.
    Brake adjustment is done on the engine side of the servo.
     
  12. Jamesk350

    Jamesk350
    Z Club Member

    The rods on the pedal/servo have adjusted/ checked by FourWays. So I’m sure that’s ok plus there was a change in the heat build up on the wheels.
    I’m going to look at the servo first as it’s a part from a scrapyard car I’ve found out so we will see soon I hope
     
  13. Turn & Burn

    Turn & Burn
    Z Club Member

    It really sounds like your trapping pressure in the brake lines, so something is pushing the m/c piston forwards out of position. There’s two things that could do this, the pin in the servo could be set too long or more likely the clevis that attaches to the pedal is set too long. As the pedal comes up against a stop at full height this would end up pushing through the servo and onto the m/c piston. I’ll attach a few pics, if you need any dimensions just ask
     

    Attached Files:

  14. stuart barrie

    stuart barrie
    Z Club Member

    You trust that fourways has set it correctly mistakes can happen.
    Was the servo changed before or after the plumber for bad been set?
     

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