[Vwdiesel] Brake boost failure....

[mark shepherd]

A few days ago during some extremely cold weather [-4 deg C] I drove off in my car and found that 100 yards up the road at a junction, the brakes went hard, and I struggled to stop. Went about half a mile before they came back to their usual responsive form. My conclusion was that maybe there was some ice on the vane pump. It happened again a few days later. Still around 0 deg C. I had driven for about 1/2 mile at around 45mph before I noticed it at a junction. Driving 10mph  for 100 yards further to the next junction and the power came back.

It seems to be the first drive of the day. It happened again this morning, but air temp is more like a chilly 5 deg C ;o)
I took the vaccum pump off and then apart, and found a little smear of sump oil, and the vanes were free to slide and were well oiled. Blowing and sucking on the plastic barb seemed to tell me that the non return valve operated.  Air freely came out of the spline base. Nothing else to see really...Reassembled and ran the car[yesterday] 

After today's reoccurance; I did a vaccuum test.
[Car had already recovered from fault at this point] 

It reached 20 to 25 inches watergauge quickly. I was attached to one of those normally capped twin little barbs.
Switching engine off gauge dropped to zero rapidly. [I put this down to either my poor connections with the gauge and being below the valve in the servo line.] 
Further to this when reassembled, the brakes retain their power for at least a minute after engine turned off. 

Suggestions welcome. [Except for comments on my misunderstanding of the term 'cold weather'] ;op

Mark