Hope this is a simple yes no question

[Rocky ST]

Can the rear brake calliper be separated leaving the brake lines in as I’m wanting to give the pistons a proper clean up & I don’t wanna be bleeding brake fluid or anything like that

 

[mjc506]

Technically yes, but you'll struggle to get enough torque on the bolts without putting the caliper in a vice. Which means removing brake lines realistically

You can give the pistons a pretty good clean with a shoelace soaked in brake fluid, 'shoeshine' style.

That said, you should really be giving the system a full flush and new fluid periodically. Some say 2 years, but in our 'damp' environment, annually is cheap insurance against SMC problems.

 

[W0QNX]

A proper cleanup can happen with a shoelace and brake fluid without taking the caliper apart or loose from the bike. The pistons will move easier than you think as in pull out just a bit. The internal area of the piston is clean.

 

[Rocky ST]

A shoelace soaked in brake cleaner
That’s a great tip for the future

 

[Mellow]

Pull the wheel would be my advise.

Article [13] - ST1300 - Rear Brake Piston Cleaning | ST1300 Articles

I was putting a tire on the bike and decided to do an article on how I clean the rear brake whenever I do a tire change. If anyone has more info to add, please post and I'll adjust this article. Okay, let's get to it. This article assumes you've already removed the rear wheel Remove the...

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[SMSW]

By now you have seen it's not a simple yes or no question.

 

[W0QNX]

A proper cleanup can happen with a shoelace and brake fluid without taking the caliper apart or loose from the bike. The pistons will move easier than you think as in pull out just a bit. The internal area of the piston is clean.

I should have stated this better. If you push on only 2 pistons the third will move out easily. Don't go to far they aren't dirty underneath. I don't move them in or out at all really.

 

[bolter1100]

As mjc506 said, most use brake FLUID and not brake cleaner. It cleans the pistons well and is a much better lubricant. Just don't spill it on any painted surface.

 

[Kevcules]

Clean the pistons as well as you can (shoelace works good) before compressing them not to push dirt past the seals. I use kerosene as a cleaner on pretty much everything. It's evaporates quickly and rubber friendly.
Remove the wheel and disconnecting the caliper makes it easy to clean it without having to disconnect the brake line.