golf mk5 brake issue

[Jfg]

Last week i had to change a seized caliper on the mrs golf. I replaced it, bled it and all was good. not even that little hook type metal pipe broke. So no leaks and all working.

Yesterday i drove it and the pedal needs to be pumped to get brake pressure. Then it sinks slowly. I have no leaks and the brakes (except the seized caliper) worked perfectly.

Any ideas?

[GreenV8S]

That sounds like a failed internal seal in the master cylinder. It is the only seal that can fail without causing an external leak and causes the symptoms you have. It's easy to damage the seal when bleeding since this takes the piston outside the normal range of travel so it can be damaged on any lips of edges caused by normal wear. You may be able to get away with a new seal but it's liable to happen again. A new m/c would fix the problem permanently.

[The Wookie]

First of all are you sure you have no leaks? When fitting a kit I’ll always start the engine to get the servo going then apply high brake pressure while an assistant looks for any weeping. Even a slight weep can actually draw air in which you wouldn’t intuitively expect.

Next give it another bleed to make sure, and if you have a mate with a VAGCOM it’s worth giving it an ABS bleed. Both times I fitted a new brake kit to my E92 (we use it for testing new kits) I’ve had to give it an ABS bleed to get rid of a long pedal, despite locking the lines off when disconnected and not running the reservoir low during bleeding. No idea why.

Failing that, as said, master cylinder. It could have been leggy and failed with giving it full strokes during bleeding