Its most likely leaking gas. The A/C is controlled by a pressure switch located inside the interior evaporator unit (and at the rear of the compressor on kouki only). If there is no gas in the unit, the compressor wont engage but the a/c and econ buttons will still light up.
It could be that the o-rings are leaking on the fittings, but its most likely that the compressor is leaking (and if its original it probably will be being 20+ years old). You can get them rebuilt, but im yet to see a rebuild kit for the rwd 4age compressors (heaps of fwd ones).