Can O valves fail?

[joetheghost]

I have three rads all with metal o valves that squirt out water through the screw surround.This is the slotted screw gets turned 180 degrees to adjust the heat.The main problem is that the rads that have them are barely lukewarm but are hot one end,where the thermostatic valve is but cold where the slave valve is,so it seems that even when the slave is opened fully, the coolant isn't getting to pass through the rad.It seems if I need to change them it will mean draining the system to disconnect them.

 

[swbjackson]

If I'm reading it right your radiators are fitted with the old style lock shield valves that operate with a screwdriver. Sounds more likely that the system is sludged and in need of flushing as well as these faulty valves changing.