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Tintop brake issue
Slimy38 - 4/12/13 at 01:03 PM

I have a SEAT Toledo (Golf Mark 4 underneath) with a very odd brake issue. Every so often when I get in it and drive off, the first braking event is met with the pedal going straight to the floor and no braking effort. It'll pump and the brakes will get back to normal for the rest of that journey, and chances are next time I start it up it'll be fine. It's only perhaps one in four startups?

I'm guessing a leak somewhere, any suggestions where to start looking? I did have to change a brake line but it sealed and bled ok.

britishtrident - 4/12/13 at 02:06 PM

Strange one, normally if it was failure it would only affect one circuit, as you don't mention fluid loss it must suggest either a master cylinder or ABS unit problem.
Only other possibility is the self-adjusters on a rear brake slipping back.
If there are no ABS or ESP warning lights coming then master cylinder is the prime suspect.

adithorp - 4/12/13 at 02:19 PM

Had this exact problem and never properly diagnosed it (mainly because it was so occational/transitory). In the end we played "parts darts" and threw a master cylinder at it. That fixed it, although we could find nothing wrong with the original.

Slimy38 - 4/12/13 at 02:21 PM

quote:
Originally posted by britishtrident
Strange one, normally if it was failure it would only affect one circuit, as you don't mention fluid loss it must suggest either a master cylinder or ABS unit problem.
Only other possibility is the self-adjusters on a rear brake slipping back.
If there are no ABS or ESP warning lights coming then master cylinder is the prime suspect.


Ah yes, of course, even if I'd completely balls up the line I'd still have 'something'.

I think the master cylinder will be a good place to start then. I remember reading something about if the pedal is pressed too far it can invert a seal, and since my wife was the one operating the pedal as I bled the brakes I wonder whether something has happened there.

Thanks for the help.

adithorp - 4/12/13 at 03:17 PM

In normal use the pistons only move a fraction of the available cylinder length. The unused cylinder section isn't as polished (even tarnished) and when you bleed the brakes using the full stroke, the seal lips can be damaged by this rough section. Anything from slight wear causing leaks or sinking pedals to a full invertion/flip giving NO brake; 90's GM master cylinders were a nightmare for this.

Our customers car was an Audi A3 so probably the same as your's.

Slimy38 - 4/12/13 at 03:32 PM

quote:
Originally posted by adithorp
In normal use the pistons only move a fraction of the available cylinder length. The unused cylinder section isn't as polished (even tarnished) and when you bleed the brakes using the full stroke, the seal lips can be damaged by this rough section. Anything from slight wear causing leaks or sinking pedals to a full invertion/flip giving NO brake; 90's GM master cylinders were a nightmare for this.

Our customers car was an Audi A3 so probably the same as your's.


Yep, that sounds about right. Even if the bleed didn't go through a full stroke, I have definitely done it since. I'm tempted to swap it out, as you say it's the most likely target for a replacement part fix.

Edit: Then again, nearly £200 for a replacement...

[Edited on 4/12/13 by Slimy38]