I have a rad in the hallway, ground floor. Left side as you look at it is what I can only describe as a valve with a y piece off it at the bottom to attach a hose pipe for draining. On the right hand side there is a valve to open and close with a + & - on the top, however, I don't seem to be able to get the top off the valve. Usually you can prise a lid off revealing a Phillips screw underneath but not this one. How do I get this off to repack/tighten the nut???