A Shrader valve is the same type of valve that is on your car tires. Shrader valves were also used with old R12 type ACs on the suction or low pressure side. Your 1995 940 is VERY unlikely to have a R12 system and in fact I'm certain it has a 134a system with 134a type valves. Perhaps your mechanic was refering to a special charging and evacuation fitting? My 1990 740 with R12 has a special fitting between the receiver and evaporator and it leaks too! Find a shop that only works on auto ACs and they'll be able to fix anything that's wrong with a Volvo AC. It won't be cheap, but this is an area of modern auto repair that's never cheap and its worth paying a competent shop to fix it right the first time for years of trouble free cold air.