These are some good information threads:
Hi looking for anyone that may have experience in removing the purge valve from an FX5. mine has started to leak, im happy to buy a replacement but unsure how to remove the old one. i dont want to pull twist or turn heavy handedly in the wrong way and make the situation worse. hopefully...
www.monsterfishkeepers.com
If its the valve itself, you might be able to lube it like the top aquastop valve, see this one too:
Looks like it's leaking from the knob. Is this an o ring fix or do I have to replace the entire valve?
www.monsterfishkeepers.com
