Originally Posted By: George7941
... and perhaps pressure helps the seal do its job ..
Well, no "perhaps" about it, George, of course the seals depend on positive pressure. And for lip seals, that clearly only works in one direction.
Think of how the master cylinder front seal works when you do a pedal-type bleed - it is specifically intended to allow reservoir fluid to pass by when a vacuum is present in the system as you return the pedal with the caliper bleed valve closed. In that instant, if the caliper had a single lip seal with the same pressure characteristics, air might enter. They use a square O-ring design that works both ways.
This particular car was an early Saab 900 and in my opinion the designers of that clutch concentric slave cylinder assumed that the system pressure was always positive and used a single lip seal (which I confirmed was present.) It worked as long as it was new and tight.
Those Saabs always had a reputation for hard shifting and I'll bet that's why.