I had a similar problem with my 91 W250.

Replaced the rear drums, shoes, etc., and the I could not get the parking brake to hold the truck. As I always considered the parking brake was an "emergency brake", my test was to drive the truck and while moving, pull the parking brake lever and hit the parking brake lever to see if the truck would stop in an emergency. It always worked before I replaced the factory original drums but not after I replaced the original drums with aftermarket drums.
While trying to adjust the parking in accordance with the FSM, I could feel and hear that the shoes were not in contact with the drum for a complete revolution. My conclusion was that the "new" drums were out of round and not machined correctly and probably should have been turned to make sure they were round. I got rid of the truck shortly after and did not get a chance to verify my theory. But if I had it to do over, I would have taken the new drums and had them chucked up in a brake lathe to make sure.