Dave do you have one of those air dryers under the truck that has the small black canister on it and the governor? if so i would put you had on the bottom of the dryer and see if there is any air slowly coming out, you can spray it with glass cleaner or soapy water to see if it bubbles and spray all the air fittings around the dryer to. If its leaking out the bottom then its your purge valve. They are not cheap however as i have done with the last 3 trucks i remove the dryers i believe its called a ADIS type and just put on a AD9 and install governor right on the compressor and run couple new syntex line or rubber braided from it to new dryer. I change my cartridge early fall and never had a problem since.
Don’t settle for just the air dryer and filter replacement if it has been neglected. The air dryer filter is a regular maintenance item such as the engine oil and filters. Make sure the air lines are flushed and cleaned out or your transmission is next to reap the consequences
Okay so I finally was able to get an answer from another driver... I've heard several trucks doing this, only the Macks. I climbed underneath and was looking around, it's set up with 2 air driers and an oversized compressor for airing off liquid trailers. Particularly sensitive oils, so no moisture gets in the air during offloading. The trucks are set up to maintain 125 PSI consistently. There's no way to adjust it. I apologize to anyone who has to park next to me on a hot day...
