I had problems with the brake regulator just behind the cab on my 77 doka. May be worth it for you to have a look with a flashlight. You can see the purge valves on the outside of the regulator look for tearing that allows water and debris in. Its a bit easier with a tipper bed tilted up.
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If you page through the photos front and back I took pictures of the regulator torn down and the rebuild kit. Its not that hard to do but there are a few small parts and springs to watch for. The problem with mine turned out to not be the purge itself (that only keeps out the water) but a piece of rust or debris. It could have come from the purge or the air line itself. It was just enough to allow the knife edge on the inner seal that holds pressure to allow air to leak by. The pressure would leak down overnight. Since I fixed it I noticed the next regulator in series is leaking I may need to rebuild that one next. It now leaks down a bit then holds like yours. Up close after shutting the truck down you could barely hear the air escaping from this regulator.
Another photo showing the location of these regulators with the bed tipped up:
The one closest to you in this photo is the one I repaired. The larger one further away is slowly leaking now. It may just need a tear down and cleaning.
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Hope this helps.
Sean