when i flush the toilet only half the water in the cistern empties?

a) check the cistern is filling to its correct level

b) it may require you flush it once, but hold the handle down, and the full flush continues.

c) looking inside the cistern, there may be a hole about halfway up the side of the diaphragm housing.

This should have a plug in in which may have dropped out. Generally these plugs are a bright different color to the other plastic items.

If so, the vacuum ends after only half the water is flushed. Try putting your finger over the hole and flushing. The plug is probably in the bottom of the cistern.
Good Luck!

Mike Newcomb
September 2010