I have recently refitted the engine to my laguna II 2002 1.9 DCI. All is fine apart from the brakes.
With the engine off the pedal is fine, as soon as I start the car the pedal is soft and goes to the floor if not touched for a short time. The pedal will go hard if you pump it but fades as you hold pressure on it.
I have bled about 1.5 litres of fluid by using the pedal and then with a vacum pump.
I have checked the vaccum pipes and the non-return valve on the servo.
With the engine running and the pedal bad as soon as I disconnect the vaccum pipe to the servo it works fine (hard, no fade). Once the pipe is reconnected it goes bad again.
I am stuck on what could be wrong. I thought maybe the servo could be leaking inside or with the added pressure of the servo working it could be pushing past seals on the master cylinder but with the servo off and heavy pressure on the pedal it does not fade at all.
Sorry to ask the obvious question but did you follow the correct left to right, front to back procedure most cars require when bleeding the brake system? Brake slave cylinders are plastic these days and have a tendency to fail causing your symptoms, the Laguna is no exception to this i'm afraid.
Currently in saabguy's garage: 2002 Citroen C5 SX 2.2 HDi