Check over on the Steel Soldiers site in the "old deuce"section. There are a few threads on the older airpac. Apparently, the majority of the repair parts are the same as the airpac on the M35 family of trucks, and there is a supplier or two who will make up the proper kit for the M135.
We have a M135 running at work where the base maintenance installed a new M35 airpac and master cylinder. The new airpacs are significantly smaller in size to the old ones, so you have to extend some lines to plumb it in. We had some difficulties with the brakes on that truck afterwards, which I recently corrected.
Note that if you use a M35 master cylinder, they have a residual check valve in them that the M135 does not have. You have to disassemble the new master cylinder, remove the check valve and re-assemble. If not, the brakes will drag. I have done a few of them now with success. The master cylinders are on ebay all the time for around $80 USD.
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