Which would you use on a sign?
A. Flush after leaving the toilet.
B. Flush up after leaving the toilet.
C. Flush out after leaving the toilet.
D. Flush / Flush up / Flush out.
I would use none of the phrases you have provided.
Rather, on a sign I would write the instructions "Please flush the toilet after use."
You would have to have very long arms to flush a toilet after you left it.