I used DOT 5 on a restoration I did on a drum-braked vehicle about 10 years ago. It was a complete rebuild so there was no contamination with non synthetic fluids.
If you use synthetic, make certain that you have ALL the old fluid out. Do NOT mix DOT 4 and DOT 5.
There were MANY early reports about synthetic fluid allowing air into the system--especially Corvette disc systems (IIRC).

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags