Reducing clauses with different verb forms

I was about to read your phone message which was asking for a ride home
I was about to read your phone message which would have been asking for a ride home

Is there a rule around what verb forms (mood, tenses, modal verbs) can be omitted when reducing a clause? I presume b cannot reduced but a can...Correct?

I was about to read your phone message asking for a ride home

Thanks