How can I fix my windscreen wash, Renault Scenic 1.6 expression Y Reg?

Hi there,
I am having problems with my windscreen wash. Twice it has been to the garage and I was told it was an air lock. If I press the button for the water to come out on the front windscreen, the water comes out at the back window. It also works correctly when I want the water to come out at the back.
I'm sure it must be something quite simple, even if it is an airlock. But how do I get it to work, without having to take it to the garage every time I run out of water?