PEH
I assume by "pneumatic cylinder" you mean vacuum actuator that drives the flap behind the center vents. It is possible to access that with the glovebox removed. I've actually replaced mine that way (on the 123) without removing the dash. It's not easy but it is possible. Obviously, if you can replace the actuator itself you can get at the vacuum lines.

Steve
If the center vents are working correctly they should open only for cold air and not with hot. It sounds like your vents are working but the vacuum lines may have been reversed at some point. When the actuator on the center vent fails it tends to work only intermittantly and/or leaks enough that you'll notice the whole system (locks, engine shutoff, etc.) working poorly. If you can hear the vent opening and closing and don't have a vacuum leak problem then I think you may be able to fix this just by tracing the vacuum lines and looking to see if they got reversed at some point. It's worth a try.

LRG

__________________
LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold