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Brake Servo Problems


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#1 oblomov

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Posted 28 November 2018 - 02:34 PM

The brakes on my Clio 172 started playing up with a soft pedal.  As with master cylinder seals failing a sharp tap on the brake pedal before braking appeared to 'set' the seals and allowed me to brake normally.  I've changed the master cylinder (and also a rear brake caliper).  However when I start the engine to bleed the brakes I still have a rock hard pedal.  Vacuum at the manifold is about 17 inHg and with the servo connected about 15 inHg in line.  15 inHg seems (from what I've read) to be the low end of what is acceptable for a servo vacuum.  So I'm wondering if the servo has failed  (very rusty down one side) or, if the aftermarked master cylinder is not suitable.  I'm now going to swap the new master cylinder for the old and see if that makes a difference.

 

Comments/suggestions welcome.

 

Thanks ob.



#2 Pidgeon

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Posted 28 November 2018 - 04:09 PM

You say you have a soft pedal and a hard pedal - which do you mean?



#3 oblomov

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Posted 28 November 2018 - 04:48 PM

You say you have a soft pedal and a hard pedal - which do you mean?


When problems with the brakes occurred I experienced a soft brake pedal. Now having changed the master cylinder the brake pedal is solid with no give at all It should soften and allow pedal travel when the engine is turned on and the servo vacuum becomes active.

#4 Johnboyhgt

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Posted 29 November 2018 - 08:55 AM

To test the servo:

 

start and run the engine for a few seconds.

turn off the engine

with engine still off pump the brake pedal around  6-7 times

with the pedal fully pressed into the carpet and held there start the engine

keep the pedal fully pressed, you should feel the pedal sink further when the engine is running.

 

if the pedal doesn't sink under your foot you have a servo problem.



#5 oblomov

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Posted 29 November 2018 - 12:10 PM

To test the servo:

 

start and run the engine for a few seconds.

turn off the engine

with engine still off pump the brake pedal around  6-7 times

with the pedal fully pressed into the carpet and held there start the engine

keep the pedal fully pressed, you should feel the pedal sink further when the engine is running.

 

if the pedal doesn't sink under your foot you have a servo problem.

Thanks, unfortunately there is no 'pumping' the brake pedal with the engine on or off.  The pedal is rock solid at all times so I'm leaning towards a servo problem because as you say when the engine is switched on the effect of the servo should become apparent immediately if it's working properly. Bugger.



#6 Johnboyhgt

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Posted 29 November 2018 - 01:40 PM

Check for a vacuum by removing the pipe then starting the engine. should be able to forcibly retain your finger on the end of the pipe.



#7 oblomov

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Posted 29 November 2018 - 08:54 PM

Check for a vacuum by removing the pipe then starting the engine. should be able to forcibly retain your finger on the end of the pipe.

 

These checks have all been done, including the one way valve on the servo, and are OK..  I've bled plenty of brake systems/changed master cylinders in my time including the VX.  My issue is that the brake pedal remains solid, but not only that, when the master cylinder is inserted into the servo the piston is depressed out into the master cylinder (spring resistance when tightening the master cylinder nuts).  This means the brake pedal is fully released and the pushrod is fully withdrawn the servo, but, on the other side the master cylinder piston is depressed into the master cylinder meaning perhaps a mechanical disconnect between the two. Presently my suspicion is that the master cylinder has not aligned centrally on the brake pushrod, which is why I can feel the master cylinder piston being pushed back iinto the master cylinder.  This is because the brake pushrod is not fitting inside the master cylinder piston sleeve.  Instead it is fouling on the edge.  A quick look at the old master cylinder which I swapped in and out again today confirms this.  On the lip of the old piston there are thread marks in one place which indicate pushrod misalignment.  A new servo is £200 and fitting requires the removal of the accelerator pedal from inside the footwell and then the rmoval of the clutch cable and then the removal of brake and clutch pedals to withdraw the whole unit and the servo from the car.  It's a PITA job so before embarking on it I need to understand what's causing the hard brake pedal.  Thanks for your input. :)



#8 Ivor

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Posted 30 November 2018 - 07:14 PM

When my abs plays up the pedal goes rock hard, tried disabling abs? Though I guess it's not on if you're bleeding brakes with the ignition off..

#9 Johnboyhgt

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Posted 02 December 2018 - 12:19 PM

So it could be a misalignment. Have you tried using a vernier to measure the push Rod depth in the servo?

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#10 oblomov

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Posted 04 December 2018 - 07:11 PM

Problem solved - thanks for all your input. :)

The pushrod end articulated and would not sit straight hence when the master cylinder piston end was introduced the apperture could not be made to allow the pushrod end to enter the  piston cavity and was fouling each time.  It needed someone to fully depress the brake pedal to bring the pushrod end to the fore of the servo allowing the servo piston to be aligned.  Then it was just a case of coordinating the relaease of the brake pedal with inserting the master cylinder into the servo body. thumbsup



#11 Johnboyhgt

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Posted 04 December 2018 - 08:55 PM

Fantastic. Glad you got it sorted.

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