That's the way to do it. Remove all the fluid that you can with the baster. Then surround the reservoir with rags and tilt it to one side and pull it loose- trying to get your finger over the opening that still has the fluid in it. Brake fluid will do a number on paint so be careful and flush with water any surface that catches any that gets away from you.

I make a strong solution of Dawn dishwashing liquid and water to soak the reservoir in before making a "scrubber" out of a piece of cotton rag and a piece of stiff wire. The back chamber is hard to clean but creative bending of the wire will get to most surfaces through the hole in the bottom. The reservoir must be bone dry before reinstalling it.

You should replace the grommets:

http://www.fcpgroton.com/product-exec/product_id/1298/nm/1985_1993_Volvo_240_Brake_Reservoir_Grommet/category_id/60

If you don't they may leak (BTDT). You will find rust under the old grommets so remove them carefully and clean the particles out even more carefully.

Lube the new grommets with brake fluid before installing them on the master cylinder and again before installing the reservoir. The reservoir installs by pushing it into position and "rolling" it with your had to get the lip under the grommet.

Randy