Could you explain "mu" (that is... the short of jemu, dative singular of the pronoun "on") as related to the translation given?
Would it translate incorrectly but literally "Were the brakes working to him before the accident"?
Yes, literally it is "to him", dative of "on", an alternative to "jemu" which is used when it is stressed. Mu is unstressed (clitic).
Did the breaks work to him before the accident? Is it possible?
I do not think so.