Came across a few pnuematic reversing relays today and I had a couple questions on them, as i couldn't seem to find any info on them, (envoiromatic is the brand). The stats were D.A. and both the chilled water and hot water valves were N.O.--- I know the purpose is to invert the "signal" pressure from the stat to the chilled water valve in this case but my questions are: Is it normal for these relays to leak a little pressure? This relay had 4 ports. The main tapped into the center port on the relay, then you have the signal from the stat tapped into another port, (can't remember off the top of my head), the chilled water valve tapped into another port and then there was another port on the relay that was unused. What would that unused port be used for?