Just to point out the obvious (or maybe not so obvious), they did not start putting R134a into US cars until 1996. If the evaporator is for R12, it may leak at the higher pressures for R134a. Reducing the system charge (by ~ 80%) is the number used in conversion kits. I know this from converting my 92 GMC truck this summer.
Back in the late 90's - and even today - there was much confusion on oils, freons, mineral oil, PAG oils, Ester oils, and how (if at all) they would work together. The kit I used this summer cost $50.00 and had oil and R134a. I pulled a vacuum on the system (using a RobinAir vacuum pump with a R12 gauge set), added the kit to 80% R12 charge, and ta-da. Cold AC. I expect there is a small leak in the truck evaporator - but can live with that.
Thinking about this, the difference in pressure between R12 and R134a in the evaporator should not be that much. But, once fluid has leaked out the evaporator pressure will rise. So a small leak starts small and appears to get bigger. Then again, maybe none of this applies to your Crown Vic.