Purple and light blue look like returns, not supplies. I think you can ignore them in your analysis.

Based on the diagrams and manuals I agree the rear brakes should work when the pump fails.

Is it possible porsche rethought the arrangement and made the front brakes the emergency loop in later cars? Typically the front brakes provide the majority of stopping power and are less likely to lock than rears, improving controllability. Seems preferable for the fronts to work if the pump failed at speed?

Agree 100%. The factory manual state that light blue is a return from the bypass valve, and the purple seems like a return to me too.

-Wayne

Hi Wayne,

This is a really dumb question or suggestion, but what about just driving the car and hitting the breaks hard. I would think you would be able to feel the brakes/wheels react in the front. I have a bump in my area going over a bridge right before a stop light, when I hit the breaks firmly slowing down before that stop light going over the bump you can feel the ABS kick in and you can really feel it in the front end, modulating the pressure. I felt this over the years in many cars with ABS. This is in my daily drivers as my SC does not have ABS but still you should be able to feel something in the front end modulate.

I guess my question is how do you know that it isn't working when you are actually driving the car? Seems like it is set up not to work when the car is not on/system pressurized.

Or just simply setting up 60 - zero cones i.e. motor week style and measure the distance. I would assume a 959 with only back brakes would stop significantly longer than one with front.

Unless, I'm missing something and you have already verified they are not working while driving.

Yes, brakes appear to be working fine when the car / ABS pump is on. The question is whether "brakes not working when car is off" is indicative of a bigger problem?

-Wayne

Wayne i Emailed you directly with my findings, but figured i would share here also.

I put the car on the lift today and had my assistant hold the brakes. All 4 held tight.
I then had him pump the brakes (car and key off)
Some slight foaming inside the reservoir, and the fronts slowly released to a point of not holding while the rears held solid.
Hope this info helps.
Question on it. Was the car already in position for the lift, was it ran before you applied brakes or had it been sitting? just wondering if it had bleed down before hand...

the light used to flash on in my 964.
it was a sender problem, in my particular case.

Good info - another data point!

-Wayne