my a/c hasen't worked since i bought the car (87' 2.8) and with it being so hot out i decided it's time to fix it and convert to r134a. the system was completly empty so i added just one can as the kit said and heard no leaks and it's holding pressure so i went and filled the system still holding pressue. the air blows somewhat cold, but not as cold as it should. i also noticed after i turn the car off a hissing noise (leak i'm sure) coming from inside of the box on the firewall (heater core area?) where one line from the evaporater goes in. it hisses for about 5 seconds then goes away. i'm wondering if anyone knows what the problem might be or how to fix it.

 

You probably still don't have enough in it.

When it's working correctly the compressor should be engaged almost 100% of the time at idle on a hot day.

Those little cans of 134 can take FOREVER to drain into the A/C system. 20-30 minutes per can not being uncommon in my experience. When it's all in there the can will be truly "empty" and light as a feather. First time I did this I was impatient and couldn't beleive how long it would take to drain a full can of 134 into the system. I probably gave up about 1/2 way through the can, wasting the rest. I always add it while the engine and A/C are running (I beleive that is the recommended procedure).

I have found laying the can on top of something hot (intake manifold) while charging helps speed the process somewhat. But it will still take a REAL long time for each can. Be patient, grasshopper!

A few seconds of hissing when you turn the car off is perfectly normal. Pressures still have to equalize between the high pressure and low pressure sides of the A/C system