Boiler drain taps on Worcester 28cdi (old type) not draining

[Daveanders]

Hi all, new to the forum, I have an old style Worcester 28cdi that I need to drain down to investigate some vibration noise. (i'll be asking opinions about that once the youtube video has finished uploading), meantime my question is; when you open the boiler drain taps, nothing happens. does anyone know if its ok to poke up the hole with a welding rod or similar to dislodge any crud that may be up there or are they fitted with a washer that I'm going to damage? They are made of flimsy looking plastic & I dont want to damage them Many thanks