Identify brake symptom

[slowcoach]

Hi all,
My brake pedal feels firm and 'bites' at the usual point (quite high up). Brakes effective as always have been. No fluid has been lost, as the level is good. But..
When I hold pressure on the pedal, it gradually sinks to the floor. If I pump the pedal , it returns to being firm. I've not had any brake work done for a while, so shouldn't be any air introduced. The whole brake fluid was changed about a year and a half ago.

I Seem to remember this is a pointer to something failing... Cylinder, servo? Or something else? Thanks !

[davidoft1]

Leaking seal, most likely a rear wheel cylinder

[ZsZ]

If no slave cylinders leak then there is some minor rust pitting inside the master cyl.

[slowcoach]

I can look at the rear cylinders, but don't think there's any loss there (level hasn't dropped)

The master cylinder does look pretty old. Changing the master cylinder looks simple enough, and high up to avoid the need for much bleed problems.


Wouldn't be the brake servo then? Do they outright fail when the time comes?

[937carrera]

Something at the back of my mind said likely brake servo fault. Check that there are no leaks from the hoses

[T25Convert]

Dropping pedal shows that the system can't maintain a pressure, so there is a leak somewhere.

If its not loosing fluid the leak must be internal - I think the only thing that fits the bill for this is the master cylinder where fluid could bleed back past the seals into the reservoir.

Servo you can test. Engine off, pump the brakes a few times (to deplete vacuum in the servo) which should give you a hard pedal. Keep the pedal pressed down and start the engine - as you start it the pedal should drop a little as the vacuum forms and the assistance from this kicks in.

[slowcoach]

Thanks, I'll do the pedal test [THUMBS UP SIGN]

[Mocki]

Master cylinder seals on the way out , happened to my early t4 exactly as you described

[itchyfeet]

Master cylinder seals on the way out , happened to my early t4 exactly as you described

this ^^^ had to do the pop top for the same fault.
often happens after changing brake lines if you bleed by pumping pedal. as the seals move past their normal travel and get damaged.

best use a pressure bledder

[slowcoach]

Servo you can test. Engine off, pump the brakes a few times (to deplete vacuum in the servo) which should give you a hard pedal. Keep the pedal pressed down and start the engine - as you start it the pedal should drop a little as the vacuum forms and the assistance from this kicks in.

Did this last night, and the pedal does drop/soften when the engine fires up, as you describe. thanks for that info guess a new master cylinder is on the cards. my next brickwerks order continues to swell

[slowcoach]

brickwerks are out of stock and im ina bit of fix (imminent holiday)-
eurocarparts have PAGID ones..
https://www.eurocarparts.com/ecp/c/Volk ... 9b4&000032

I know theyre good for brake parts, is this a decent make of cylinder? I don't want to fit junk

[937carrera]

No specific experience, but Pagid usually tends to be decent stuff. However their clutch cables for Polos are utter cr*%

[itchyfeet]

brickwerks are out of stock and im ina bit of fix (imminent holiday)-
eurocarparts have PAGID ones..
https://www.eurocarparts.com/ecp/c/Volk ... 9b4&000032

I know theyre good for brake parts, is this a decent make of cylinder? I don't want to fit junk

I fitted Pagid on mine works as it should

[slowcoach]

Thanks fellas

[slowcoach]

urghh waste of time. went to collect the part from ecp, paid first at counter. the part appeared and looked like it had been kicking around the floor for years, box smashed up, cylinder was oxidised, and had no rubber bungs in. I explained I wasn't going to fit it and got a refund. no appologies, and "that'll take 2-3 business days" to refund back to me.