im a bit late to the party but, yes they are prone to failing, not exactly blocking. for a ten to thirty quid its definatly worth replacing. they usually fail by unblocking themselves and allowing too much oil into your intake.

they are a spring actuated diaphram thingy. they prevent engine vacum from sucking in too much oil into your intake but allow case pressure to be vented into the intake if needed.

they also minimise your oils exposure to external moist air, by sealing off your head, they help prevent condesate from forming in your engine.

you can clean it out but most likely if its been on there since factory the amount of oily crud and crusty rusty spring goop will be nie on impossible to get it to function properly again, the rusty spring will no longer hold pressure.

you can clean the pipes with a rag on a stick or use teapot/win bottle brushes. make sure your clean them thoroughly.....you dont want 20 year old rusty oil spring goop running through your engine.

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