Supposed to allow air through until certain pressure and then it closes. Basically the opposite of over pressure valve. It allows motor to start without having to build pressure for a few revolutions.
Pretty easy part to replace. Google should find you many.
I dont care about karma but i do like to see a users history, something i apparently cant do on mlem. Not sure if thats site-wide or just this app. Without that i wouldnt be able to confidently use the buy-sell communities that i did on reddit like knife_swap or watchexchange.