"Square" radiator drain-off cock not working

[manville]

Our three downstairs radiators are all fitted with this type of drain-off:

http://www.builderdepot.co.uk/radiator-draw-off-cock.html

But short of removing the valve core (which is effective but messy) I cannot figure out how to get it open? I have turned it both ways without effect on two out of the three radiators. What am I missing? Having discovered and fixed a leak I need to top up the inhibitor in the system. I thought I would run some Fernox F3 through it first so I need to be able to drain down the system to refill it afterwards.

If it's just blocked would connecting the other end of the hosepipe to the mains be effective and what do I need to consider to prevent damage to the rest of the system? (I have a very old boiler)

 

[Phill Peck]

That type of drain cock are ****e.

As u say only way to use them can be messy. Towels down and a tub under neath if possible. Set up your hose to out side remove the core and get the hose on the end as quick as possible.

Stupid idea they are

 

[dilalio]

Get yourself a brand new, heavy duty rubble sack "with no holes (apart from big one at top ) and use that as a wrap around container to catch spills whilst you unblock it!