pierogi

I seem to be having issues with the a/c system in my 1991 LS 400. The A/C hasn't been checked in the 7 years I've owned the car and was working flawlessly until one day it felt warmer than usual, the next day it was blowing hot. I've hooked up a pressure gauge to the low side and found a reading of 45-50 psi with the engine off. With the A/C running (the compressor turns on) I then get a reading of 0 psi. A/C code displayed is 33. Does this indicate anything, a refill is needed?