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Brake pedal sinks

Discussion in 'Ibiza Mk4 (2002-2008)' started by hidnmine, Nov 13, 2019.

  1. hidnmine

    hidnmine Active Member

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    The brakes work really well on my Ibiza, but if I keep my foot on the brake pressing hard, the pedal slowly sinks towards the floor.
    There is no fluid loss, so I assumed the master cylinder seals must be leaking. I've now changed the master cylinder and it's exactly the same..
    I'm lost. What else can it be??
     
  2. mgrays

    mgrays Active Member

    Joined:
    Dec 9, 2006
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    First you need to separate the brakes from the servo or Booster. So switch off engine and pump the brakes about 5 times.. you will find it gets harder as you pump and seemingly the pedal will get higher (but it is a trick.. I will not go into why). Anyway.. now you will have exhausted the servo of vacuum so it will be only the brakes i.e master cylinder, valves, fluid and calipers. So now leave it 20 seconds and then give it 3 really good pumps. You should not notice the "bite" point or maximum travel changing at all. If it is different then try to work out if it gets shorter travel on each stroke = air in brakes, or if it sinks as you hold the force on = leak in brakes or inside master cylinder. Then hold the pedal down and restart the engine; the pedal should appear to sink as the servo kicks in with the assistance or boosting. So not sure when you have the pedal sinking.. it might be a leak in the servo supply pipework or vacuum getting lost as if the engine is off .. might be just overthinking the brakes.. try another car to see how it reacts in the same treatment.
     

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