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
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.