406 Brake booster malfunction

[Farmpug]

Hi all,
We have a problem with the brake booster on our 406 HDI 2.0 Estate (or "waggon" here). The boost function started to get intermittant and now will not work at all. The brakes still work fine - you just have to push really hard. The pedal does not creep down when pressed so I believe it is not a hydraulic problem.
How is the vacuum to the brake booster controlled?
What is the source of the vacuum?

Thanks,
FarmPug

[Farmpug]

Ah the thot plickens.
A few more observations:
1. The brakes are very hard at first then after a couple of seconds, work as usual. Yes it is very challenging to drive smoothly!
2. If you put your foot on the brakes while stationary with the engine running the pedal will slowly sink to the floor.
3. If you repeat this with the engine stopped the pedal stays firm and does not sink.
4. The engine performance has reduced considerably - like it has no boost.

I have checked the vacuum pump by testing for vacuum at the brake booster and this seems OK (although I have now found a spec of 500mm Hg after a couple of minutes at idle)
I would normally believe that the sinking pedal would be caused by the master cylinder seals failing (there are no fluid leaks anywhere)
BUT, why do I only get a sinking pedal with the engine running?
and what has the sluggish performance got to do with any of this?

Regards
FarmPug

[Farmpug]

Hi,
Problems both fixed. The booster had a big air (vacuum) leak, this was enough to prevent the brake booster working initially and enough to increase the control pressure for the turbo control.
Removing the booster revealed the rear seal was parted from the body of the booster. It is apparently repairable at moderate cost (about 1/3 of new here)
I could not get a s/h one to repair nor an exchange unit so I replaced it for new.

Removing and refitting the booster is a job best avoided. It is possible without removal of the cylinder head or engine as one manual suggested!

Regards,
FarmPug