backroom boy vs brain truster

backroom boy and brain truster both are nouns.

Word	Noun	Adjective	Verb	Adverb
backroom boy	Yes	No	No	No
brain truster	Yes	No	No	No

As nouns, backroom boy and brain truster are synonyms defined as:

backroom boy and brain truster: an expert adviser involved in making important decisions but usually lacking official status

backroom boy (noun)	brain truster (noun)
an expert adviser involved in making important decisions but usually lacking official status	an expert adviser involved in making important decisions but usually lacking official status