hope i don't get cracked too bad by asking this question but, other than color and size what makes each check valve so different that they are not inerchangable. does it really matter which one is hooked up to each line. do some hold more pressure/vacume than others?why one metal on and the rest plastic??? just wondering

 

Ya, I think the only difference is how much they can hold, but you can swap them and everything will still work fine.