why is water coming from the pressure relief device and not the shower head?

Hi I have a Triton t80 easi fit shower which is leaking from the pressure relief device? I've had a look and the pipe and it is clear, I've change the pipe to the shower head and with out the shower head on the water runs free but when I replace the head it comes from the pressure relief even when I put a new shower head on?

andrew
November 2020