How can you tell if gear box breather valve is working on mowers and brush cutter?

[MAArcher]

The valve plug on my land pride fdr2560 and my brush hog are looking rusty. Wondering if there’s a way to test if they still work?

 

[cthomas]

Remove plug and blow thru it with your mouth.

 

[Flintknapper]

Remove plug and blow thru it with your mouth.

^^^^

This.

Low tech method works fine.

 

[MAArcher]

Should air pass through freely or is there an actual valve and it open with a certain amount of pressure like a car thermostat?