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Slightly modern content…. 05 Skoda Octavia workhorse, but applicable to many VAG things. Had issues with binding rear brakes caused by sticking calipers. Calipers (alloy bodied) had corrosion under the seals so we have renewed these, twisting off the short link pipes in the process. These have been remade. Flexihoses were clamped off so fluid loss was minimal. Problem now is that whatever we do, we cannot get a decent pedal. Pedal feels great with the engine off but goes 1/2 - 2/3 to the floor as soon as the servo comes in. It can be pumped up but then sinks under sustained pressure to about the same point. Calipers are not over-retracting (bit draggy if anything) and the pedal feel was previously good. Bleeding (gravity and two-person method) has been done in situ and with the calipers dismounted to try and encourage any trapped sir pockets out. This did help a little, but not enough. Clamping the rear flexihose on either side considerably helps the pedal feel, implying that there is still air in the calipers, but no more wants to come out! The pedal sinkage is a concern. To me this suggests the bleeding has done the seals in the master cylinder. However there are several tales on the VAG forums of sinking pedal being unfixed by renewing the master. And it looks like a very nasty job…… Any thoughts /wise words? Have pic of an interesting and unusual VAG product for your trouble…. Nick  |
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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zeberdee
Club Retro Rides Member
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Strange brake behaviourzeberdee
@zeberdee
Club Retro Rides Member 2
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Did you fit new brake pads at the same time as the callipers ? Could need bedding in ??
The pedal dropping with the engine running seems to be a modern diesel thing .
Feel it in loads of different cars , but the brakes always give good results on the brake tester & on road test .
Think this could be because they use a vacuum pump rather than the vacuum from the inlet manifold ??
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Last Edit: Oct 30, 2022 8:37:30 GMT by zeberdee
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Dez
Club Retro Rides Member
And I won't sit down. And I won't shut up. And most of all I will not grow up.
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Club RR Member Number: 34
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Strange brake behaviourDez
@dez
Club Retro Rides Member 34
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How is the car positioned when are you bleeding it? You need weight on the axle, not with the wheels dangling free as it’ll shut the brake compensator. It could be trapping air there.
It *could* be the master cyl seals don’t line the extra stoke from bleeding, but id try the former first.
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zeberdee
Club Retro Rides Member
Posts: 933
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Strange brake behaviourzeberdee
@zeberdee
Club Retro Rides Member 2
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Just to add , i drove a VAG vehicle with faulty master cylinder seals , it didn’t start to brake until the pedal was almost on the floor .
You’ll definitely know if the master cylinder is faulty .
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How is the car positioned when are you bleeding it? You need weight on the axle, not with the wheels dangling free as it’ll shut the brake compensator. It could be trapping air there. It *could* be the master cyl seals don’t line the extra stoke from bleeding, but id try the former first. Thanks Dez. No weight on the axle, but it’s abs equipped so won’t (I don’t think) have a compensator. No problem getting fluid through. Aware of the risk from stroking the master - prefer to gravity/pressure but gravity wasn’t doing it and no pressure bleeder available. Cheers Nick |
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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Did you fit new brake pads at the same time as the callipers ? Could need bedding in ?? The pedal dropping with the engine running seems to be a modern diesel thing . Feel it in loads of different cars , but the brakes always give good results on the brake tester & on road test . Think this could be because they use a vacuum pump rather than the vacuum from the inlet manifold ?? 👍 Same pads and disks. Seem to sit snug together. A little snugger than I would like actually with more drag than I would like, but that’s a different problem. This sinking pedal thing with modern stuff is odd. First encountered on Nissan XTrail. ABS system related?? Nick |
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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Rich
Club Retro Rides Member
Posts: 6,239
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Strange brake behaviourRich
@foxmcintyre
Club Retro Rides Member 160
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Oct 30, 2022 11:21:28 GMT
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How is the car positioned when are you bleeding it? You need weight on the axle, not with the wheels dangling free as it’ll shut the brake compensator. It could be trapping air there. It *could* be the master cyl seals don’t line the extra stoke from bleeding, but id try the former first. An 05 Octavia is likely to not have a compensator as it’ll probably has ABS. EDIT: OP confirmed this 😂 |
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Last Edit: Oct 30, 2022 11:21:49 GMT by Rich
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Dez
Club Retro Rides Member
And I won't sit down. And I won't shut up. And most of all I will not grow up.
Posts: 11,712
Club RR Member Number: 34
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Strange brake behaviourDez
@dez
Club Retro Rides Member 34
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Oct 30, 2022 11:27:06 GMT
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How is the car positioned when are you bleeding it? You need weight on the axle, not with the wheels dangling free as it’ll shut the brake compensator. It could be trapping air there. It *could* be the master cyl seals don’t line the extra stoke from bleeding, but id try the former first. An 05 Octavia is likely to not have a compensator as it’ll probably has ABS. EDIT: OP confirmed this 😂 Yeah it’s about the time they started to disappear. Having ABS is no marker for not having a compensator though- I have numerous vehicles with both. |
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Oct 30, 2022 13:19:32 GMT
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That’s interesting, I would have thought ABS makes the compensator valve redundant but maybe it stops the ABS kicking in too soon when lightly laden. I know my A6 Avant definitely has no compensator. It does (did) have restrictor orifices at the rear caliper connections though never really discovered why. Cunningly made from aluminium so they fail totally and dump all the fluid leading to a “brown moment”. These are now deleted with no detectable difference in behaviour.
Anyhow, back to the Skoda. Took it up the road somewhat gingerly, and actually, while the pedal is a little low and squashy, the brakes work well enough. Got a bit braver, found a muddy lane and gave the abs a bit of a workout, which seemed to help a bit.
Have gravity bled again since getting home and got a small bubble from each side. Usual driver reckons only a little bit lower and squashier than before now, so as the rear calipers are now actually working, I reckon it’s near enough. Still sinks under sustained pressure with the engine running, but seems, bizarrely, that this can be considered normal range. Will keep a wary eye on performance and fluid level and hope it behaves. M/C access is horrible.
Thanks for comments and thoughts.
Nick
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1967 Triumph Vitesse convertible (old friend)
1996 Audi A6 2.5 TDI Avant (still durability testing)
1972 GT6 Mk3 (Restored after loong rest & getting the hang of being a car again)
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mk2cossie
Club Retro Rides Member
Posts: 2,948
Club RR Member Number: 77
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Strange brake behaviourmk2cossie
@mk2cossie
Club Retro Rides Member 77
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Oct 30, 2022 20:31:50 GMT
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All I can add, is are both handbrake levers on the calipers against the lock stops when released? Had loads of VAG group stuff with the cables poorly adjusted, which doesn't allow the caliper internals to properly adjust and operate  As for the pedal creep, usual behaviour for a lot of slightly more modern stuff, that has a servo ratio that easily overcomes the master cylinder operating pressure 😳 |
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Oct 31, 2022 11:15:29 GMT
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If pressure bleeders aren't an option, vacuum bleeding may be.. A honkin great big syringe (as big as you can find, really) and a bit of hose onto the nipple, pull the plunger out to draw fluid in. If it's taking a while you can wedge something under the plunger to hold it out and have a cuppa or mess with the pedal while you're waiting.
Or use the syringe to back-fill the system through the nipple.
Despite being fairly simple on paper, I've always found hydraulics to be more of a dark art than an exact science.
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Davey
Posted a lot
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