My 1997 960 has no high side connection, and the indie Volvo shop that works on my A/C system told me that it isn't needed. The only way to have the correct refrigerant charge is to drain the system, measure the amount pulled out, and re-charge with the correct amount. If the amount pulled is low, there's a leak.

The system in my car has vacuum controls for air direction, including directing air through the evaporator or not. There is a vacuum reservoir and a check valve to keep the vacuum strong enough during times when the engine is not making enough.

Test your system. At a stop, when the hot air "takes over", rev the engine to 2000 rpm and back off the gas quickly. This will raise the manifold vacuum. If the air flow changes to cool and the hissing changes, your problem is in the vacuum system.

The hissing sound is likely a vacuum leak. Rule out the check valve. Old age makes rubber connectors go bad, and those are very suspect. You may have to get up under the dash and listen for the hiss.

Good Luck,

Bob

:>)