Mr Gee

I seem to have a problem with one of my water jets (for front windscreen) If I go for a drive, the jet seems to leak - mind you, only a few droplets. This does not happen when the car is stationary. Only when I am on the move. If you start the car and leave it idleing, this does not happen. I have checked all the connections and they all seem fine.
Is there some sort of valve inside the jet itself that has gone wrong.
Am only assuming its that particular jet as the other one is OK.
I have tried with a full water tank and also a low level one. It makes no difference.
Anyone have any ideas ?