My RNA concentrations from brain tissue were at highest around 80ng/mL and as low as 5ng/mL. Can I converse my RNA to cDNA with this low concentration of RNA?

Per ml? or microliter?

If you mean ml... then I think this is too low. In case you are talking about microliters, then 5nm/ul is still really low. You could attempt precipitation, but it also depends on the volume you have this amount in. I would always attempt using the same amount of RNA, so you may want to dilute the 80ng to 50ng in a reaction, and then using 50 ng of the 5ng/ul by taking a lot of volume (10 microl of RNA in your reaction). I am not entirely sure if that will be high enough for all subsequent procedures, but I think it could work. 
It is pretty low though, but I think it is worth trying, especially if you cannot get any new RNA from brain tissue.

Maybe Charon can verify whether or not this really is the case.