Triton T90 si -airlocked-

Manual here
http://www.tlc-direct.co.uk/Technical/DataSheets/Triton/T90Si.pdf

KILL POWER BEFORE DOING ANY WORK ON AN ELECTRIC SHOWER!

Check page 3 to find out where the bleed screw is. Open this and wait till you have a good flow of water coming out.
Close screwn and replace the cover. Turn on the power to the shower

Turn all dials back to cold setting and turn on shower. (I also keep the shower head below the main unit) and it should be working in a few seconds.