There is no adjustment on the pony brake that I am aware of. There was a post a while back that contained some good tips regarding this problem. Try to use the search feature to find a post titled "Pony Motor Engage". It is about a D4 but the information will also apply to your machine. It talks about different weights of oil you can try in the clutch compartment as well as some adjustments that can be made.
No.9 Auto Patrol 8A331
No.9 Auto Patrol 8A539
Minneapolis-Moline prototype crawler X253
1945 Farmall H
1923 Fordson on Trackson crawler conversion