Heater Core - Reservior ??

If I'm considering installing a heater core/Radiator from an 84 Chev Cavalier do I really need to have a reservior? After all isn't in a vehicle don't they act as the reservior?? If I do need to have a reservior, why??

A res is kinda nice to have, if you set it up right,
it is a fill/ air bleed point in ur system.
I made 1 from 2' pvc pipe. works fine,
I made a better one that fits properly into my case tho.
Now I can close my case.

Res's are not necessary. A "T" fitting will work just fine.
I prefer to have some capasity in my sys tho.

just my .02