Find a nice steep hill.
Aplly the park brake going forward: establish effectiveness of the PB by itself.
Then, release the PB, apply the brakes hard before setting the park brake. Repeat test.
relese the PB again.
then Apply the parking brake and then hit the pedal hard repeat test.

Either should result in better contact but one may work better than the other. Testing on the same hill will identify if either method improves the effectiveness

might also try applying the parking brake for short periods while moving to help arc them to the drum. Do allow adequate cool down periods and obviously don't overheat them.