Before jumping in and replacing the recirc diaphragm, test the vacuum manifold behind the control unit.
The recirc diaphragm may well be intact and the leak may be elsewhere. Heck, it may even be just the heater valve (easily reached under the airbox - has a white vacuum line to it, and if it fails, you get hot all the time).
Go read John Pirtle's write-up on testing the vacuum system including the vacuum lines for each HVAC actuator..
928 S4 Service - John Pirtle
In both my S4's, the orange line (comb flap) had a leak, and in the '89 the heater valve had failed too - all others were fine including the recirculation valve (which is a pig to get to). A leak in any of the diaphragms will cause loss of vacuum and heat.