1986 Tercel with Air conditioner problem

[armyjunk]

Hello, Maybe someone can help with this problem, My AC seems to work most of the time. Sometimes at low speeds it quits working and puts out hot, humid air that fogs the windshield. It has been converted to 134. It brings the air down to around 60 degrees, when working, on a 90 degree day. When engine is shut down it has an odd hissing sound coming from either under the dash or the firewall area. It sounds like it is bleeding air through a valve. Thank you for any ideas or help! 1986 Tercel Wagon.

Roger