Brakes seized on mid journey

[GreenV8S]

PomBstard said:

Considering it’s age, that’s our best guess too, just haven’t seen these symptoms of failure before.

I've seen it on two different cars.

[PomBstard]

I’d not seen it in nearly 35 years of driving, owning and working on various cars. And the specialist had never seen it in about 40 years of working on front engines Porsches. But now we have

Anyway, pic to show why I’m at least pleased it wasn’t the servo - just behind the master cylinder…

[GreenV8S]

You're unlikely to see it on cars that are well maintained with correctly adjusted brakes. It's those scary DIY jobs where somebody has been looking after it with good intentions but insufficient knowledge where you'll see problems like this.

[Scrump]

Glad to hear you have it sorted.
That picture of your 928 is a bit of a tease! Can we see the whole thing please?

[PomBstard]

Scrump said:

Glad to hear you have it sorted.
That picture of your 928 is a bit of a tease! Can we see the whole thing please?

Thanks, and whilst it’s entirely the wrong thread, as you’ve asked nicely…

[The Wookie]

Just to explain what was probably the mechanism, the port to the reservoir would have been blocked (either through distorted seals or possibly marginal adjustment/tolerance that had crept into the wrong position through wear).

Basically the brake lines become sealed leaving nowhere for the fluid to go. The pads/fluid/etc warm up when you apply the brakes and they expand, taking up any free travel in the system and in turn creating more residual pressure in the system until the point where the pads are in contact with the disc and the brakes are dragging. At that point the fluid gets really hot, the pads get really hot and it creates a vicious cycle where the drag rapidly turns into a significant amount of brake pressure and the car grinds to a halt.

Not uncommon in racing cars where it's easy to cock up the master cylinder free travel length on an adjustable pedal box.

The comedy aspect can come after the marshals have dragged the car off the track with it's wheels locked... You're stood leaning against the car having a chat with them waiting for the race to end so you can be recovered back to the pits... the brakes cool down, eventually they release by themselves, and you end up sat on your arse wondering what the hell just happened to look up and see the marshalls running after the car to stop it rolling back across the track.

Ask me how I know this.

Not heard of it on a 928 before though I must admit!

Edited by The Wookie on Friday 18th June 10:14

[Dave Brand]

The Wookie said:

The comedy aspect can come after the marshals have dragged the car off the track with it's wheels locked... You're stood leaning against the car having a chat with them waiting for the race to end so you can be recovered back to the pits... the brakes cool down, eventually they release by themselves, and you end up sat on your arse wondering what the hell just happened to look up and see the marshalls running after the car to stop it rolling back across the track.

Ask me how I know this.

. . . or, when the driver says "I just lost drive" don't rely on leaving it in gear!

[DarrenO'D']

My Quattro had exactly the same issue about ten years ago, the master cylinders just wear and will not let the shaft return fully. Somewhere around the same time a few others had the same issue, turns out 30 years was about the age it happened.

[slybunda]

brake hose went on mine and acted like a one way valve making caliper stick on. lucky it happened about 20meters away from my house.