Bendix AD IS Purge air leak

I've been searching the web, looking for as much information as I can get on the Wabco compressor. Or anything else that might help me on this. Apparently, this isn't a common problem. I actually went to a dealership and was told to change the desiccant filter and purge valve. I had my doubts at the time, because the purge valve wasn't leaking at all while charging, or when the engine was shut off. But I did what he said, it didn't work so I bought a whole new dryer and nothing changed. don't think the problem is with the dryer itself. I'm thinking I'll have to take it a more qualified mechanic.

 

Ok thanks for all your efforts. I crawled under the truck trying to find an air leak. I can hear something very very small. I haven't pinpointed it yet. I have a couple of very small airleaks on the fire wall, but they've been there for while. I keep a spray bottle with me on the truck, but it's not a magic cure all. Space is very limited on these 579's . I'll have to cut away 20 or 30 zip ties and do some digging around. I've had ringing in the ears for 40 years, so that further complicates things. I'll mess with it tonight for awhile. Start out fresh in the morning if I can't find anything tonight

 

I didn't notice it pulsing before, but yeah I just checked it. It is pulsing like the compressor

 

Hey, I got my truck back from the dealership a couple of days ago. First day, they couldn't figure it out. Checked for leaks, found none. Second day, they called Bendix and was told that a Bendix dryer would fix the problem.
Bendix said the internal components on the after market dryer weren't calibrated as precisely as the Bendix. (Yeah right) It didn't fix the problem. They tested the compressor and found that it was functioning properly. Called Bendix again and was told to replace the main air line from the compressor to the dryer. (At that point what else could it be, right?) Bingo. Problem solved. Air line was about $130.00 .
I had sprayed that line looking for a leak and so did the guys at Kenworth. I didn't see any bubbles, but apparently something about the old line was bad. I had considered replacing it early on, and being a cheapskate, It came back to bite me.