hissing from front and rear A/C

I'm getting a hissing noise from the front and rear A/C. The hiss starts when I turn the a/c on and gradually fades out when I turn the A/C off.

If I let it run it becomes intermittent and the A/C is not very cold

thanks

I hooked up some gauges to my A/C and Low pressure reading was 20 and High side was 200, I was told that these A/C units are just about the worst ones on the market and actually require a higher pressure than the manufacture lists. My mechanic suggested I put up to 55 psi on the low side and see what happens. I added another can of R134 and the air from the vents dropped from 80 (ambient temp) down to 60, a second can dropped the temp to 47 degrees. and the low side pressure went top 35. So far so good and the hissing sound has gone away.

Camflightrn