you do not say what make the shower is, i know, that the redring showers have a pressure release valve, it is fitted at the top of a clear plastic tube, you have to remove the cover of the shower and, you can see that at the top of the tube, a brass nut you undo this, and inside the mounting there is a rubber valve [it normally looks like a bowler hat] if holed it has to be replaced / it goes when the shower head is blocked and pressure builds up /clean shower head to prevent it happening again
frank, August 2011
