Unloader Valve Lines

Hi, I have a Ingersoll-Rand air compressor Model 15T/Serial 30T 904746. It stared out building slow air pressure in our plant but when I unhooked the line off the top of the heads that runs to the unloader valve it builds up fine (3 lines feed together to the valve). Is there supposed to be a little pressure coming out of that line when the compressor is running or is that a problem with something in the head. Right now I have the line disconnected and everything is fine....is that going to hurt anything?? Whenever I hook it back up it makes a muffle sound out the intake/air filter and build slow again....seems to me like they are working against each other. Any help would be much appriciated!!!! Thanks, Jake Naperville Illinois USA