From The Third Degree, published in 1969 by Arno Press. The period referenced is the 1920s:
Policemen in Boston must pay their own fines and judgments when convicted or sued for lawlessness. There is a benefit fund for policemen’s widows and orphans and for policemen injured in the performance of their duties. but there is no protective fund maintained out of dues and contributions which can be used for judgments or fines…The policeman’s money liability is said to create a frame of mind that stands in the way of false arrest or the use of club or fist.
This simple measure from the past (along with today’s smartphones) would take all the fun out of police brutality.