Harry MTrucks

Hi need help mechanic is busting me , my entire air syst seems to be contaminated with brownish black type of dirtish oil and its cgging my hoses ruining my braks system and the tanks are all contaminated , do I need a new compressor or is a filter ? the mechanic cleaned the air tanks and they were full of this material , and the air systems is constantly getting clugged ... what coud it be I feel the mechanic is cleaning but not fixing my main problem help!1

I own a fleet and have had some bad luck with a couple of mack rb 1999 electrical wanted to know whats the truck model that least breaks down and takes a lot of hard rough work a strong car I can only buy around the 1994 -2000 range cause of my budget. Please help . I own a DM is done pretty good just would like some suggestions buying a new fleet soon. also is it normal for trucks to be breaking constantly or a good trucks should be a ble to take a beating before it eventually needs fixes?? Thank you