crowizcom wrote:

however, brakes are very weak, and if i apply excessive force, there
comes metallic sound ..

What Andre Jute said-- your brake needs grease. Since that is a very
unusual and extreme condition for a grease, I would take Shimano at
their word when they say you must use their special roller brake
grease.

Occasional greasing is the only maintenance that roller brakes need,
but they really need it.

Chalo