Most drum brakes do not function as well when being forced in the reverse direction. It's just the way they are designed. I'd double check the shoes are on correctly (larger shoe on the rear) and the adjusters are not babackwards but your probably just experiencing the the effects inherent to how drum brakes work. When being tourqed in a forward direction the shoes are forced harder into the drum, this does not happen when being tourqed in the opposite direction so your left with alot less braking power. My mustang has 4 wheel drums, it barely even slows down when in reverse lol.

 

 

Yes, most people will notice that if they forget to release the Parking brake they can back out of the driveway but can't go forward and that's when they realize Parking Brake is on, lol.

So Parking brakes don't hold as well from rolling backwards, on a slight hill they are fine but not on steeper hills, you need to add Park or engine compression, 1st or Reverse gear on manual trans.
Not a design flaw just the only way to make drum brakes work, i.e. you need more stopping power in a forward direction

And always turn front wheel so it will hit the curb if truck does roll