scott is offering the best advice in this thread with one exception, you should not use ATF in the p/s system of the 124 car. Since the time that car was built, ATF went through some drastic changes and will attack the seals and other rubber parts on your p/s system.

Follow scotts method, but use p/s fluid from the auto supply store.

If you're not going to flush the system, changing the filter and replacing only the fluid in the reservoir is just a waste of time.