I would just bleed it out with new fluid. You're opening a can of worms messing with that old of a system with "other" chemicals, especially if you want to use it in the near future. Just bleed the brakes like you normally would and you won't have to worry about air in the system either. Just my 2 cents,
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!