The BMW E34 Air conditioning circuits are filled with either R12 (1989 - 1992) or R134a (1993 - on).

See A/C upgrade for more information about switching from R12 to R134a. PDF File for Condenser exhange.

The A/C system is split into two sides, a high pressure side and a low pressure side.

You need to discharge your circuit for any work on the A/C system. Local A/C shop will give you a credit if the R-12 is recovered. I don't recommend to evacuate the circuit yourself with a vacuum pump because of the environment concerns.