The most comon one is the fresh air flap, which can be replaced with the dash pad installed. So can the defroster flap. There is one inside the case that requires removal of the dash. I've never replaced all on a 201 so I dont know how many there are, I beleive its 5 total. But, dont hold me to that.
According to my data, there are 4 inside plus the heater control valve under the cowl. The main vacum in is behind the boosteron the checkvalve/splitter make sure youve got good vacum to that point, also check the heater valve for leaks, either from under the cowl or from the switchover valve.
#1 main vacum in.
#3 and #4 blend air flap "cold"
#5 heater control valve
#6 defroster short stroke
#7 defroster long stroke
#8 legroom flap closed
#9 fresh air flap short stroke
#10 fresh air flap long stroke (recirc door)
Thats the layout for a 90 model 201 w/acc. I'd check #1, 5, 9 and 10 first, however, check 'em all. Aterall the switchover valve is already in your hand. Just be sure to remove the lines one at a time for testing, otherwise you might fix the leak but nothing will do what you want.
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