brake failiure on 2002 Volkswagen Passat

I have a 2002 Volkswagen passat estate, diesel.
Recently the brakes completely failed while driving in ice conditions
-20°C on a mountain road (no servo assistance) as I descended into warmer air they started working again.
The Volkswagen garage said that there is a join in the vacuum line that can collapse (where it passes through the engine compartment bulkhead) & they inserted a short section of rigid plastic hose into it. This seemed to work until a few days later when again at about minus 7°C I put on the break & they locked on, the pedal also stayed depressed. I found that I could pull the pedal back up with my foot slowly & the breaks would release. As soon as the temperature goes above minus 2°C the breaks start to work perfectly. Also when this happens there are no fault lights that come on.
Any ideas? Cheers.

Asked by for the 2002 Volkswagen Passat
1 answer
this can occur if there is water in the brake booster. drainage issues in the plenum area , usually caused by leaves blocking the drains , allows water to accumulate and enter the vacuum booster. freezing conditions obviously turn the water into ice , which will jam the booster/brakes internally. warmer temperatures thaw the ice relieving the blockage.
you need to replace the brake booster after confirming this diagnosis and repairing the root cause of the problem , water in the plenum.