Camera input / output resolution w/ postprocessing

I'd like to be able to render my scene to a lower resolution and then render it to full resolution with some postprocessing applied .. but I can't find a good way to do this.

I've tried rendering my scene to a rendertexture and use that rendertexture in a second camera & apply postprocessing there, but for some reason when I run the game my rendertexture gets removed automatically from my first camera!

No warnings or errors are given :(

(I have unity pro)

Anybody have some pointers here?