Answer:
Always old, sometimes new. Never sad, sometimes blue. Never empty, sometimes full. Never pushes, always pulls.
You get many of me, but never enough. After the last one, your life soon will snuff. You may have one of me but one day a year, When the last one is gone, your life disappears.
By Moon or by Sun, I shall be found. Yet I am undone, if there's no light around
My first master has four legs, my second master has two. My first I serve in life, my second I serve in death. Tough I am, yet soft beside. Against ladies cheeks I often reside.
It flies when it's on and floats coming off.
I build up castles. I tear down mountains. I make some men blind. I help others to see.