Back in September of '16 I replaced my Haldex air dryer with a factory rebuilt. The old one had partially failed allowing some desiccant powder to get loose in the system, but because there wasn't anything malfunctioning I didn't go "powder hunting".
When I replaced the dash AC vacuum generator under the steps last fall I noticed a bit of powder, but nothing serious and the auxiliary compressor comes on only about 4 times a day (24 hours) and runs for about 1-2 minutes each time, then shuts off. I have
sniffed around looking for leaks, but haven't found any, and from what I've read here on the forum I'm satisfied that my system is in pretty good shape.
In the past I would get an almost indiscernible amount of white powder when the bleed valves were opened, and after installing the new Haldex I got none. None that is until the other day when I opened the wet tank bleeder and got a puff of desiccant powder, I think. My eyes ain't what they used to be so it could have been water vapor, but nothing has been emitting since the new Haldex.
I can't believe the new dryer has failed in 4500 driven miles, but when it quits raining I'll have a look.
Since I never removed and cleaned the wet tank, maybe it's residue?
I'll bet on residue from what you have said. Without having cleaned the tank, some residue had to have been in there.
Agree - it is most certainly "residue" from previous operating conditions. Once that cr@p gets in your air tanks and lines, you will never completely get rid of it. Search "desiccant" on this Forum. Jim McNeece, Barry Beam and Michael & Jackie have told horror stories of trying to clean out the air system after drier cartridge failure. Short of replacing the wet tank with a new one, there is no way to completely eliminate it. Hopefully, it will eventually "set up" in the bottom of the tank, and not cause any other problems.
Residue will coat the inside of the air tanks and stay there for a long time. Use a Boroscope to look inside and maybe be surprised.