Civic a/c compressor

I put a used compressor on a 98 civic and when the a/c is off, pressure is over 100psi. (low side). When a/c is on, pressure falls slowly and sometime I have speed engine up get it to 45psi. Do you think compressor is weak or bad expansion valve?

Your low side pressure should fall when the AC is running. Just like the high side pressure should go up. Is the air getting cold??