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Welcome to the Outfit!
First place to start troubleshooting would be to crank the truck, let it build air - then listen from under the truck. If you happen to have a shop air compressor, attaching a glad hand and air line to the front driver's side will air up the truck with no noise. If you don't hear air leaking then you need to break out a spray bottle of soapy water. The CTIS hoses (possibly behind the "hub caps") need to be sprayed. All those hoses under the truck may need your attention too...
The TM's for 939 Series trucks are here for downloading:
A couple of these may be repeats but I'll include them anyways in the interest of trying to make applicable TMs, TBs, LOs, and MWOs easier to find. There are only a couple missing and I'll notate those in hopes that someone else may have them and can link them as well. Please understand that...
www.steelsoldiers.com
Since the last post was being a real PITA and didn't like having so many links in it I had to break this into 2 posts. TM 9-2320-272-24P-1: PUB DATE 08/27/2012; REPAIR PARTS AND SPECIAL TOOLS LIST (RPSTL) FOR TRUCK, 5-TON, 6X6, M939A0/A1/A2 *SUPERSEDURE NOTICE - This manual supersedes TM...
www.steelsoldiers.com
Lots of reading and looking with lots of troubleshooting flowcharts are hidden in these books.