Hi,

My name is Fabian and lives in The Netherlands, Europe.

I have the following problem with my Porsche 928 s4 from 1991

I hope I can explain in english wat happens !!

If I want to brake , directly the brake goes down and i don,t have any pressure anymore, if I will put my feet of the brake and directly put my feet on the brake i will feel some pressure but very fast again the brake goes down and no pressure at all, so the brake dosn,t work.

There is enough Brakefluid so I i think there is no leak in the cables/cilider ?

i have asked some people and they said, maybe the problem is due to the brakemaster cilinder or maybe the ABS pump ?

I hope somebody understand what my problem is and maybe can help me.
In a few days i will go to the USA for vacation maybe I can buy directly the part what i need, that brakemaster cylinder is partnumber: 928.355.011.24

I hope to get some info from you guys.

Greets,

Fabian from The Nethelands

check the back side of the master cylinder where it bolts to the vacuum booster unloosen the nuts and see if fluid is leaking out.( if you see fluid then its bad)
Check the whole system for a leak you may have a cut line.
Also how old is the master if its original and the fluid hasnt been changed every 2 years then a master is probably bad

If you don't have a bad flex line to a caliper, it's surely the master cylinder. I would REALLY doubt it is the ABS hydraulic unit.