If I perform a coolant flush on a 03 maxima/i35 will the reservoir drain with the radiator or am I better off buying a siphon to get rid of old resivor coolant?

 

the reservoir does not drain with the radiator. vehicles with surge tanks (pressurized coolant reservoirs like GM commonly uses) will drain with the radiator, non-pressurized coolant reservoirs will not drain with the radiator.

best thing to do is unbolt the reservoir, dispose of the coolant, clean it out with hot tap water, bolt it back on and put new coolant in it

 

just remove the container as shown in this video and empty it out manually.