Axle breather

Hi All
I have read a few posts about axle breathers but most seem related to extended ones with tubes not the one I have (see pic). I have a leak from the diff/shaft seal so I am replacing this seal at the weekend. I have read that a dodgy breather can exasperate a leak (pressure finding the path of least resistance?) So I took off the breather and tried to blow through it from the thread side but I couldn't, can you test them this way???
the offending item is currently soaking in carb cleaner

Iirc there is just a ball bearing in there, and yes you should be able to blow through them.

Thanks Lynall

All I can see is rust inside, so will get a new pair

They also get gummed up if you waxoil the underside.