you forgot the magic word lol <img src="emoticons/icon_smile_approve.gif" alt="Approve"> just kidding. First, you'll want to go here:
https://www.logsa.army.mil/etms/ind...e2291170e-D5D25FB2-E46F-8BDD-CDC2E7A8708EC4BC
and look up the following pub numbers in that search area and download them:
TM 9-2320-361-10 operator's manual
TM 9-2320-361-20 maintenance
TM 9-2320-361-34 maintenance
TM 9-2320-361-34P repair parts and tools
TM 9-2320-361-20P repair parts and tools
or just search for M35A2 or M109 in the pub title text and all the available manuals will come up
air filters just need to be cleaned, they're reusable, and the first fuel filter is very hard to find or unavailable. You'll need to remove that and clean it, but be careful cause then you'll need to bleed the fuel system.
also bank some money on new batteries, it's unlikely they'll be able to fire having sat so long.
Change the oil, all 22 quarts of it <img src="emoticons/icon_smile_approve.gif" alt="Approve">
then to the brakes- does he have any clue why they don't work, or did they just "quit"? You'll need to find out what happened before he decided to park it. Do you know if the air system holds air, or did when it was parked?