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It sounds like there may be a restriction in the system, possibly at the expansion valve at the inside in the evaporator housing, this is why there is this hissing noise, there should be no noise. The high pressure line (the small diameter) should be hot, the larger diameter line is the low pressure side which should be very cold, not ambient temperature, this why it is not getting cold. The normal pressure while the compressor is running should be about 28-35 psi on the low and the high side around 150psi.
If it still uses the R-12 freon it's going to have to be retro fitted for the R134 type which is the only freon available anymore,