brake problem

my 300tdi auto has an unusual brake fault. when you apply the footbrake all is well, but if you have to apply the brakes again quickly (eg if your pulling out of a junction and a car is coming quicker than you thought) the pedal goes rock hard and no force is applied to the wheels. the servo and pump seem fine, if you pump the pedal then start the engine with the pedal applied, the pedal sinks a little.
i'm confused with it and when it happens it scares the beejesus out of me. any ideas?

ABS or non-ABS?

ABS system doesn't have a servo. If ABS then check pressure accumulator and/or pressure switch. How often is the pump running?

If non-ABS then likely to be a vacuum leak - check vac hoses and vac pump. Failing that possibly need a new servo?

HTH

I had just suffered from this and I replaced the servo and all is well. The brake appeared to work but then the pedal went as hard as a rock and braking effectiveness was greatly reduced. I do not have ABS BTW

the abs has been removed. just fitted new discs, pads and callipers as the rears were siezed, and a piston on each front calliper was siezed. i thought that was the issue.