See previous post on seized handbrake for adjustment on handbrake "drum in hat" disc type. This procedure would be similar for models with rear drums only & should be performed first.
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https://www.fiatforum.com/ducato/225989-handbrake-stuck.html
The following are basic instructions there are also other adjustments that can be made.
With the vehicle on the ground - can be done with the wheels on the ground if you can fit under the vehicle - also a lot safer.
With the handbrake lever in the rest position, unlock the locknut (1d). adjust
the front cable adjustment nut (1a) until the lever (1b) just touches the "end of travel position" on the bracket (1c);
With the handbrake lever still at the rest position.
1.Then tighten the front adjustment nut (1a) till the inner cable end is just contacting the lever when at the end of travel position. (see bottom pic) then tighten locknut. (1d)
2.With that adjusted apply the handbrake lever to see how much travel the lever has. Then using the the lower illustration in the top pic adjust the rear cable adjuster (1) so that when the handbrake is applied it only moves 3-4 notches on the ratchet.
Tighten the locknut (3).
Refit the rear wheels. (if still removed) Remove the vehicle from the lift.
Check that the handbrake is working correctly & lever travel is as above.