Only when you take the rear callipers back down to their vest & underpants do you fully understand how they work & unless you have the correct tools to rebuild them you’ll be wanting replacements.
The chamber in which the needle rollers live is originally filled with a specific grease & over time this degrades & water gets in, turning rollers into rust dust. If you squirt any type of spray “oil” in you create an rust/ lubricant paste, this dries out rapidly & solidifies exacerbating the problem further.
Prior to the squirt of death some of the rollers remain in place by dint of the rust around them, wash this out & you have a problem multiplied.
I’ve no doubt that many folk have been squirting there callipers (not a euphemism Cobber) & getting away with it but eventually - - - - -
M

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