STRANGE AIR LEAK...NEED HELP

i never thought about it leaking out of the body of the compressor. that may be the issue. last night when the truck was leaking down, i pulled the air filter off of one of the compressors and air was leaking out, which means the smc check valve that is 6 months old (a replacement for the leader hose check valves) wasn't doing its job, which is rediculous. so i just plugged the inlet and the tanks still leaked. got up this morning after airing the tanks back up, checked the same compressor inlet and no air was leaking. so if that is the problem, then how could a check valve work for a couple weeks then fail, then work again, etc.? that makes no sense. has anyone even had the big smc check valves that SD sells fail, i wasn't expecting that. it should last longer than 6 months for the $18 or whatever i paid for it. are there any other check valves that may work better?

is there a correct way to install a check valve, like does it matter if it is laying horizontal or vertical? does one way help keep water or moisture from getting in it and keeping it from closing? just trying to figure out why my check valve works sometimes but not others...

bump^^^