In what years has the Chicago Teachers Union gone on strike since
1975? How long was each strike action?

Hello.

Here are the years and length of all Chicago Teachers Union strikes since 1975:

1975                  11 days 
1980                  10 days 
1983                  15 days 
1984                  10 days 
1985                  2 days 
1987                  19 days

source:
STRIKE HISTORY
Chicago Sun - Times.: Sep 10, 1993.  pg. 6

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There haven't been any strikes since 1987.

" Chicago public school teachers have rejected a proposed five-year
contract, potentially setting the stage for the first teacher's strike
since 1987. "
source: CBS 2 Chicago
http://cbs2chicago.com/topstories/local_story_290103956.html

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I hope this helps.

Thanks