No brake boost when (very) cold - 87 4KQT

[Quattrospg at aol.com]

With the recent frigid weather I seem to be experiencing a short period 
without vacuum assistance on the brakes. Pedal stays firm with little travel. Once 
the car warms up a bit all is well. Only happens in really cold weather and 
when the car is cold soaked. 

Any ideas? My only guess is that the check valve is freezing/sticking, not 
allowing the engine to pull a vacuum on the booster until heat from the engine 
can free it up.

Thanks in advance

Bill
87 4KQT