The AC on my 1987 745T started giving trouble two weeks ago. The dash vents would close and the AC would go to the default mode of defrost and heat would come out when going up hills when the engine was in the boost range.

The Volvo engineers figured if there was a vacuum failure that defaulting to defrost, for a clear windshield was important, so that is why the dash vents quit with vacuum failure. Heat starts coming out of the defroster vents with a loss of vacuum because there is no vacuum to hold the water valve (for the heater core) shut.

One of the tips in the 700/900 FAQ was to inspect the rubber cap on the right hand side nipple of the vacuum reservoir, which I did. The cap was completely split. $2.19 US bought three vacuum nipple caps. I used one on my car, and the other two will be installed shortly as preventive maintenance on other cars in the fleet of four.

Many thanks to Steve for the 700/900 FAQ!