I have an electric shower which is playing up. Whenever I turn it on water comes out of the pressure release hose and also leaks out of the main casing. The only way to stop it is to remove the shower head. This led me to believe that the shower head was scaled up so I descaled it but the problem persists. Related to this is that when the shower head is removed the water that comes out of the hose contains small flakey particles. What I think is happening is that these particles build up inside the shower head causing the backward pressure, BUT, what is causing the particles in the first place? The shower is at least 8 years old, more likely 10 years.
The hose could possibly have a rubber inner lining which is perished and disintegrating and the bits are blocking up the shower head thus causing the back pressure.
Try a new hose and shower head, they can't be too expensive, can they??