Larry and Ted,
My father was a mechanic for 40+ years. I learned about cars from him.
Even though there was a special tool in the tool box for pressing caliper pistons, the 'jumbo channel-locks' have always been the tool of choice for easy compression. I too have never had problems arise from the use of channel locks for brake work. Between my father and I, we have been doing it for almost 70 years in total!
Three things my tool box will never be without... big channel locks, a big hammer, and a long screwdriver. If the 'special tools' do not work, one of these will.
Back to the main question, do I need to replace both rear calipers or can I get away with one for now and the other later? Would it be a good idea to replace all four? Sort of preventative maint. before the front ones fail. Could the previous owners lack of maintenance have damaged the master cylinder?
I have had this car for 5 years and only flushed the system three times myself, the last one was almost 2 years ago. I have learned my lesson, the brake system starts regular, anual flushing starting now!
76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles