What purpose do these serve ?

[Gregor]

This solid cylindrical part that sits within the rear brake piston what’s it for ?

Never noticed it before, the front brake pistons don’t have them.

[mangocrazy]

Does the centre section move outwards when the rear brake is pumped? I've seen similar on cars when there's either a handbrake piston, or on certain 'Superduplex' systems where there are two brake fluid circuits operating different (concentric) pistons in the same caliper.

[Gregor]

Does the centre section move outwards when the rear brake is pumped? I've seen similar on cars when there's either a handbrake piston, or on certain 'Superduplex' systems where there are two brake fluid circuits operating different (concentric) pistons in the same caliper.

No, the centre part doesn’t move independently, this is off my yamaha xt660z.

[Skub]

I always assumed they were to help slow transferring the heat from the pistons to the brake fluid.

Dunno if that's true or not,just makes sense to me.

[Rockburner]

I always assumed they were to help slow transferring the heat from the pistons to the brake fluid.

Dunno if that's true or not,just makes sense to me.

That would make sense.

I always thought they were just load spreaders.

[Le_Fromage_Grande]

It's to keep the shit out of the hole

[Skub]

I always assumed they were to help slow transferring the heat from the pistons to the brake fluid.

Dunno if that's true or not,just makes sense to me.

That would make sense.

I always thought they were just load spreaders.

It's to keep the shit out of the hole

Could be all those things.

[David]

Don't think it's a load spreader...Methinks the heat answer was more likely....reduced surface area = slower dissipation.

[A_morti]

What that lot said.
Also:
It's a Brembo caliper with aluminium pistons, I think? So maybe to prevent wear on the piston from the steel pad.
Reduce vibration/squeal.

[ZRX61]

They're isolators

[Ssray]

Isolators, for heat and squeal