528e A/C Recharge

I have a 88' 528e that I am trying to recharge the a/c system. I have yet to determine which is the low pressure point to attach the refrigerant cans. I tapped the one on the compressor, as one book I had mentioned this was the low pressure tap. Pressure was low, then went up to 110lbs. So I switched to the tap up under the coolant tank. Pressure was high, then dropped to about 70lbs. The spot window to see the coolant in the drier shows nothing but bubbles when a/c system is on. It is supposed to be 116F tomorrow, so any help would be great.
Thanks

Highly recommend seeking professional help with your a/c, esp. if your not sure how to ID the piping!
If the system was low or empty, it'll have air in it, and on and on and on.......I DON"T BELEIVE A/C WORK IS A DOoIT YOURSELF PROJECT, unless your
perhaps in the HVAC field and know the systems..jb.