Tails

Sab,
Don't forget to change the brake fluid in the PSB as well. It should be changed every 2 years.
Tails 1990 928S4 Auto

Sab

Thanks, don't you need a Bosch Hammer to flush the PSD system?

Bill Ball

Unless you were doing a lot of heavy braking and boiled the brakes I would not expect brake fluid quality to play a role. Most likely a bad master; something like a seal that fails to seal some of the time. Otherwise, hard to explain. I suppose it is possible there is an ABS defect mode that could do this. If the ABS thinks a wheel is locked up, it opens a valve that returns fluid to the reservoir and lowers brake fluid pressure. Farfetched but possible.

blown 87

Here is what happened, one side of the master cylinder was already bad and you did not know it, the second side lost the seal for a short while and you had no brakes.

Simple as that.

You can and should flush it, but you must put a new master cylinder on it before you run into something.

GregBBRD

It is the only thing that could be intermittent. It has pistons with rubber seals that touch the bore of the master cylinder. This is what moves the fluid. If one of the rubber edges doesn't seal, it doesn't push fluid.

Tough to believe that both "rubber seals" in the master cylinder could fail at the same time....that is the only real question.

Sab

I had master cylinders fail before and there is still some resistance when pushing on a bad master cylinder. There was no resistance what so ever when it failed and now it is perfect, I can stand on the pedal with all my weight and it will not give at all. Solid pedal...

blown 87

I am thinking the back side was all ready bad.

Tails

You can bleed the PSD without using a bosch hammer.

There are 2 writeup available, one on Jim Moorehouse's CDs an from PCA.

There has been links to PCA's write up on this forum, so suggest you do a search.

Tails 1990 928 S4 Auto

GregBBRD

Try pushing really gently...almost like playing with the pedal with a toe...up and down very softly.

DeWolf

That sounds like an ABS issue. I have had this happen on two cars. A Ford Falcon( Oz family sedan) and a 2000 XJR. Turned out to be a crap in the abs pump or whatever it is in the Jag. As for the Ford, who knows, I sold it cheap to someone and told them it had abs issues.
Modern stuff is great, until it isn't.

Forgot to add that as soon as I let off the brake and re-applied they worked.

Brent

Here is the link: http://928oc.org/journal/psd1.pdf
Did it last weekend and installed the new reservoir since I couldn't see where it had ever been done. Also helps if your left handed.

Sab

OK, mystery solved...
Was able to reproduce the problem again in extreme heat. The brake fluid was old and exceeded it's boiling point.

Vacuumed out the brake fluid in the reservoir and it was nasty. It looked like it may have never been changed before. Cleaned the reservoir and flushed the entire system and all is good now. Nice firm pedal and no problems even once the brakes are hot and I get into traffic...

Jfrahm

if the brake fluid was overfilled you might lose brakes in heavy traffic.

GregBBRD

How?

Why?

Jfrahm

It expands and causes the brakes to drag. I suppose it's supposed to overflow or vent somehow but that does not always work.