What you are doing is incredibly dangerous, but I do not believe that you know why or how so I'm going to attempt to paint a picture for you here .
The brake wheel cylinders are absolutely the weakest part of a Deuce, or any air over hydraulic brake system for that matter, they have a higher failure rate than any other part on that truck .
By using a rod to push on the brake pedal constantly, you are loading every wheel cylinder on that truck all of the time that it sits parked, and at least one of them is going to fail, this is not a maybe, it is guaranteed .
When it does fail there's a probability that it could start and drive off or just roll over some hapless person or yourself or a child and then your life will change for the worse .
Out of all the the things on your truck that you could jury rig to get by with, this isn't one you should fool with .
I guarantee you somewhere on the world wide net or possibly even somebody on this site has some brake shoes available, and If you're not mechanically apt you could find someone to put them on for you .
There is also a parking disc brake kit that goes on the differential pinion that I've seen available online .
In the meantime please either park your truck on the level and use wheel chocks, you will never ever live to regret being safe .
And a bus load of nuns thanks you in advance .