Lack of air assist but brakes working would point to the air pack not doing its job.
It can be rebuilt or replaced. Brakes not working could also be a bad master cylinder.
Especially for people not very familiar with the trucks I recommend downloading, reading and following the Technical Manuals ("TM") which have all the illustrations and step by step instructions you would ever need.
It takes a bit of effort up front to get used to the TM, but it is well worth it.
TM-9-2320-209-20 is the Organizational Maintenance series, so it has a bunch of different volumes that it's listed under.
TM9-2320-209-20-1 is the Maintenance Schedule
-20-2-1 and -20-2-2 are the Org. Maintenance Troubleshooting
-20-3-1, 2, 3, and 4 are the actual Organization Maintenance repair manuals
-20P is the parts manual
You'll probably want to look in both the Troubleshooting manuals and the actual repair manuals. They are all listed on the "Deuce TM's" page on this site or you can go to
https://jatonkam35s.com/jatonkaM35sTMdownloadpage.htm
and get them there.
Also, searching on this site will unearth a treasure of information.
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