My 2005 t&c was low on freon this spring. I filled the system and checked for leaks, replaced leaking seals at rear evap, under right rear, filled up system. About 2 weeks later, the a/c blows warm again and can just hear a hissing noise in heater case, in both front and rear, similar to the noise heard when shutting van off and pressure stabilizes. Any help would be appreciated, thank you

 

They sell leak detecting freon with a dye that show up under black light. I have used that stuff and found a leak using my kid's black party light. If you want to spend more money you can buy an electronic leak detector. It's a sniffer thing that beeps annoyingly until it finds a source of refrigerant, then it beeps really annoyingly.