Problem with Thomas Compressor

I've already tried using the search deal for this but have come up with nothing so I decided to just start a new thread. I've noticed my tanks have been taking longer to fill so I pulled my compressors out, the little shitty thomas ones and discovered one of them is somehow blowing out the exhaust like it's supposed to but at the same time it's blowing air back out the intake. Anyone have any idea how or why it's doing that and how can I fix it??? I already know I need new/ better compressors but I'm trying to get some other stuff done first.