Would behålla work here?
Could this also be translated as, "He usually saved her letters."?
What makes brev plural here?
If it's just the one letter, brukade doesn't really make sense.
1 ) Han brukade spara hennes brev :-
He used to save her latters
2 ) Han brukade behålla hennes brev. :-
He used to keep her letters
Could spara have anything to do with spare? Like you keep something to have spares? Or am I searching too hard to find common words?